ELECTROCATALYTIC AND ANTIBACTERIAL APPLICATIONS OF SANDWICH 
TYPE POLYOXOMETALATES 
Except where reference is made to the work of others, the work described in this 
dissertation is my own or was done in collaboration with my advisory committee.  This 
dissertation does not include proprietary, restricted or classified information. 
 
 
_______________________________________________________ 
Anand Venkatesh Sankarraj 
 
 
 
 
 
Certificate of Approval 
 
__________________________ 
Vince Cammarata 
Associate Professor 
Chemistry and Biochemistry 
 
__________________________ 
Curtis Shannon, Chair 
Professor 
Chemistry and Biochemistry 
 
 
 
__________________________ 
Wei Zhan 
Assistant Professor 
Chemistry and Biochemistry 
 
 
 
__________________________ 
Thomas Albrecht-Schmitt 
Professor 
Chemistry and Biochemistry 
 
   ____________________________ 
 George T. Flowers 
 Interim Dean 
 Graduate Schools 
ELECTROCATALYTIC AND ANTIBACTERIAL APPLICATIONS OF SANDWICH 
TYPE POLYOXOMETALATES 
 
 
 
Anand Venkatesh Sankarraj 
 
 
A Dissertation 
Submitted to 
the Graduate Faculty of 
Auburn University 
in Partial Fulfillment of the 
Requirements for the 
Degree of 
Doctor of Philosophy 
 
Auburn, Alabama 
December 19, 2008 
 
 
 iii
ELECTROCATALYTIC AND ANTIBACTERIAL APPLICATIONS OF SANDWICH 
TYPE POLYOXOMETALATES 
 
Anand Venkatesh Sankarraj 
 
 
Permission is granted to Auburn University to make copies of this dissertation at its 
discretion, upon request of individuals or institutions and at their expense. The author 
reserves all publication rights. 
 
 
 
       ______________________________ 
 Signature of Author 
 
 
 ______________________________ 
 Date of Graduation 
 
 
 
 
 
 
 
 iv
VITA 
 Anand Venkatesh Sankarraj, son of Mrs. Maheswari Sankarraj and Dr. S. 
Sankarraj was born on Dec 4, 1978 in Shencottah, Tirunelveli, Tamil Nadu, India. He is 
the eldest in the family with one younger sister, Dr. Anuradha. He obtained his high 
school diploma from Mahatma Montessori Matriculation School, Madurai in 1996. He 
gained admission into Central Electrochemical Research Institute (CECRI), a renowned 
national laboratory in India and graduated with a Bachelor of Technology in Chemical & 
Electrochemical Engineering in the year 2000. After a brief stint in the Chemical 
Engineering Department at Auburn University, he entered the Chemistry & Biochemistry 
Department at Auburn to pursue his Doctoral Degree in Analytical Chemistry in the year 
2003 under the tutelage of Dr. Curtis Shannon. 
 
 
 
 
 
 
 
 
 
 v
ELECTROCATALYTIC AND ANTIBACTERIAL APPLICATIONS OF SANDWICH 
TYPE POLYOXOMETALATES 
 
Anand Venkatesh Sankarraj 
Doctor of Philosophy, December 19, 2008 
(B.Tech., Central Electrochemical Research Institute,  
Madurai Kamaraj University, 2000) 
 
 
164 Typed Pages 
Directed by Curtis Shannon 
 
The primary goal of this dissertation is to study the structure-property relationship 
of sandwich type polyoxometalates and to employ these versatile molecules in 
electrocatalytic applications such as oxygen generation and reduction. An attempt has 
been made to design novel catalysts by a systematic variation of the type of the 
substituted transition metal in the sandwich type polyoxometalate framework.   
Chapter 1 presents an introduction to polyoxometalate structure and chemistry 
with a detailed literature review of the value added properties responsible for the catalytic 
activities of these POMs. A brief insight into the phenomenon of electrocatalysis is 
provided and the suitability of employing POMs as electron transfer mediators is 
discussed in detail. 
 
 vi
Chapter 2 is a detailed outline of the synthesis and structural characterization of 
sandwich type polyoxometalates. The novel transition metal substituted sandwich type 
polyoxometalates were synthesized and characterized by the following analytical 
techniques - FTIR, Raman, UV-VIS, elemental analysis and single crystal XRD. 
In chapter 3, a detailed discussion on the electrochemical properties of sandwich 
type polyoxometalates was provided along with some preliminary electrocatalytic 
applications. 
 Chapter 4 deals with the study of electrocatalysis of Ru
2
POM for oxygen 
evolution reaction. In-situ detection of oxygen generated at the gold working electrode 
was performed with Clark oxygen sensor and rotating ring disc electrode studies. A 
detailed description of the measurement of oxygen concentration is provided. 
Chapter 5 presents the application of electrochemical surface Plasmon resonance 
spectroscopy in studying the adsorption of sandwich type POMs onto gold electrodes as 
an elegant complementary technique to electrochemical quartz crystal microbalance 
method (EQCM) to probe the interfacial processes.  
Chapter 6 represents the successful application of transition metal substituted 
polyoxometalates as co-catalysts for oxygen reduction reaction at several noble metal 
electrodes, such as gold, palladium and platinum. A simple thermodynamic model has 
been propounded as the basis for the choice of bimetallic catalysis.  
Chapter 7 deals with the application of sandwich type polyoxometalates as 
antibacterial agents for poultry based pathogens such as Salmonella typhimurium and 
Listeria monocytogenes. 
 vii
ACKNOWLEDGEMENTS 
I am deeply indebted to my advisor Dr. Curtis Shannon for being such a great 
motivator and a generous human being, whose penchant for surface science and 
electrochemistry has fostered a conducive environment for creative research and 
development. He has been a pillar of support during the many trials and tribulations of 
my professional career. I would like to offer my sincere gratitude for his many thoughtful 
suggestions, both at the technical and personal level. I would like to acknowledge the 
immense contributions from my committee members, Dr. Vince Cammarata, Dr. Thomas 
Albrecth-Schmitt and Dr. Wei Zhan, in the direction of valuable suggestions and helpful 
insights in aiding the course of my research.  Dr. Vince Cammarata and Dr. Eric Bakker 
were two of the best teachers during my graduate study at Auburn. I will always hold in 
fond memory, their dedication to teaching and remarkable ability to impart knowledge to 
the next generation. Dr. Albrecht-Schmitt had been a constant source of support in 
providing valuable technical inputs for my research and his group?s help in XRD 
characterization, has been a launching pad for my work. Dr. Omar Oyarzabal of poultry 
science at Auburn has been a very encouraging presence to provide execution to my anti-
bacterial applications. My professors at Central Electrochemical Research Institute 
(CECRI, India), Dr. K.R. Murali, Dr. Asokan and Dr. John Berchmans have been 
responsible for setting up the foundation for my research career. 
 viii
I have received so much support from the faculty, administration assistants, and 
friends at the Chemistry Department at Auburn. I am very thankful to Mr. Tom 
Carrington for his helpful suggestions and assistance in setting up analytical 
instrumentation. My colleagues and friends in our research group have been very helpful 
at various stages of my study at Auburn and I would like to thank each one of them, Dr. 
Tsunghsueh Wu, Dr. Ugur Tamer, Dr. Yuming Chen, Dr. Lunsheng Zhang, Dr. Murat 
Alanyalioglu, Dr. Serdar Abaci, Ms. Junxua Xin, Mr. Chaokang Gu, Ms. Sridevi 
Ramakrishnan, Ms. Rajakumari Ramaswamy, Ms. Hongxia Zhang, Ms.Weiping Li, 
Ms.Tanyu Wang, and Ms.Yajiao Yu. I am very thankful to the help provided by Dr. Tyler 
Sullens, Dr. Travis Bray, Dr.Tatiana Shvareva and Ms. Anna Nelson in XRD analysis of 
my single crystals. Ms. Kauser Hussain has been very helpful in providing 
microbiological expertise and assistance for my work. 
I would like to pay tribute to all my friends at Auburn, who have been more than a 
friendly ear or an encouraging word, Mr. Karthik Narayanan, Dr. Shankar 
Balasubramanian, Dr. Vivek Krishnan, Mr. Raghu Viswanathan, Mr. Narendhiren Pari, 
Ms.Kavita Arumugam, Mr.Varun Rupela, Dr.Pradeep Prasad, Dr.Ramkumar Krithivasan, 
Mr. Gopinath Balasubramanian, Mr.Eswar Sabapathy, Dr. Jitto Titus, Ms.Kauser 
Hussain, Mr. Tony Jefferson, Mr.Rajan Panchapakesan and Mr. Ranjit David. My mentor 
and good friend, Mr.Sankar Narayanan deserves special mention in shaping the course of 
my life and being a beacon of hope & encouragement to many more youngsters in India.  
I would like to thank my parents and sister for their many sacrifices and for being 
extra supportive of my education since I was a kid. They have been a source of 
encouragement and unconditional love and I dedicate my work to their indomitable spirit.
 ix
Style manual or journal used: Journal of the American Chemical Society 
Computer Software used: Microsoft Word 2003, Microsoft PowerPoint 2003, Microsoft 
Excel 2003, Origin Pro 7.5 and Endnote 9
 x
TABLE OF CONTENTS 
LIST OF TABLES............................................................................................................ xii 
LIST OF FIGURES ......................................................................................................... xiii 
1. INTRODUCTION ...........................................................................................................1 
 1.1 Motivation for research...............................................................................................1 
 1.2 Background of electrocatalysis...................................................................................2 
 1.3 Types of electrocatalytic mechanism..........................................................................4 
 1.4 Polyoxometalate (POM) ? Fundamental introduction................................................5 
 1.5 Sandwich type polyoxometalates..............................................................................10 
 1.6 Electrochemistry of POMs ? Structure/property relationship ..................................12 
 1.7 Similarity to enzymes and porphyrins ......................................................................15 
 1.8 Important value adding properties ............................................................................16 
 1.9 References.................................................................................................................18 
2. SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF SANDWICH TYPE 
POLYOXOMETALATES.................................................................................................20 
 2.1 Introduction...............................................................................................................23 
 2.2 Materials and methods ..............................................................................................23 
 2.3 Synthesis of sandwich type polyoxometalates..........................................................23 
 2.4 Structural characterization ........................................................................................28 
 2.5 Single crystal XRD Analysis ....................................................................................33 
 xi
 2.6 References.................................................................................................................38 
3. ELECTROCHEMICAL CHARACTERIZATION OF SANDWICH TYPE 
POLYOXOMETALATES.................................................................................................40 
3.1 Introduction...............................................................................................................40 
3.2 Literature review.......................................................................................................41 
3.3 Materials and methods ..............................................................................................44 
3.4 Results and discussions.............................................................................................45 
3.5 References.................................................................................................................64 
4. DIRUTHENIUM SUBSTITUTED POLYOXOMETALATE AS AN 
ELECTROCATALYST FOR OXYGEN GENERATION ...............................................68 
4.1 Introduction...............................................................................................................68 
4.2 Materials and methods ..............................................................................................72 
4.3 Results and discussions.............................................................................................70 
4.4 Conclusions...............................................................................................................82 
4.5. References................................................................................................................84 
5. ELECTROCHEMICAL SURFACE PLASMON RESONANCE OF 
POLYOXOMETALATES.................................................................................................87 
5.1 Introduction...............................................................................................................87 
5.2 Theoretical background on surface plasmon resonance ...........................................88 
5.3 Electrochemical adsorption of Polyoxometalates on electrodes...............................90 
5.4 Materials and methods ..............................................................................................92 
5.5 Results and discussions.............................................................................................95 
5.6 References...............................................................................................................105 
 xii
6. POLYOXOMETALATES AS Co-CATALYSTS FOR OXYGEN REDUCTION....108 
6.1 Introduction.............................................................................................................108 
6.2 Materials and methods ............................................................................................112 
6.3. Results and discussions..........................................................................................113 
6.4 References...............................................................................................................124 
7. ANTIBACTERIAL APPLICATIONS OF SANDWICH-TYPE 
POLYOXOMETALATES...............................................................................................127 
7.1 Introduction.............................................................................................................127 
7.2 Antibacterial properties of polyoxometalates .........................................................128 
7.3 Toxicity...................................................................................................................130 
7.4 Materials and methods ............................................................................................131 
7.5 Results and discussions...........................................................................................134 
7.6 References...............................................................................................................144 
8. CONCLUSIONS AND RECOMMENDED FUTURE WORK..................................146 
 
 
 
 
 
 
 
 
 
 xiii
LIST OF TABLES 
Table 2.1 Analytical Data for Na
n
[ZnWM
2
{ZnW
9
O
34
}
2
] .................................................29 
Table 4.1 Literature Overview of Ruthenium Polyoxometalate Synthesis with Respect to 
the nature of precursor .......................................................................................................71 
 
Table 4.2 EDX Analysis of single crystal specimen of Ru
2
POM......................................73 
 
 
 
 
 
 
 
 
 
 
 
 xiv
LIST OF FIGURES 
Fig 1.1 Schematic of mediated electron transfer .................................................................5 
 
Fig 1.2 Ball and stick model of sandwich-type polyoxometalate......................................12 
 
Fig 2.1 Flowchart for the Synthesis of Ruthenium Substituted Polyoxometalate .............25 
 
Fig 2.2 Schematic representation of the central metal belt depicting the lattice  
position, where the transition metal substitution occurs during the formation of  
sandwich type bi-Keggin structures...................................................................................26 
 
Fig 2.3 Transmission FTIR data for solid sandwich POMs in a KBr pellet. 
Na
n
[ZnWM
2
{ZnW
9
O
34
}
2
], where M=Ru, Ir, Ni, Mn........................................................31 
 
Fig 2.4 Raman spectroscopic analysis of single crystals of Zn
2
POM and Ru
2
POM.........31 
 
Fig 2.5 UV-Vis of sandwich compounds...........................................................................32 
 
Fig 2.6 ORTEP plot (ellipsoids at 40% probability) of the X-ray structure of 
[Ru
III
2
Zn
2
(H
2
O)
2
(ZnW
9
O
34
)
2
]
14-
.........................................................................................35 
 
Fig 2.7 ORTEP plot of the X-ray structure of Single Crystal Ir
2
POM..............................37 
 
Fig 3.1 Cyclic voltammogram of 0.2mM Zn
2
POM in pH=5 buffer solution, with  
Glassy Carbon as Working Electrode (W.E), Pt mesh as Counter Electrode (C.E) and 
Ag/AgCl as Reference Electrode (R.E), at a scan rate of 10mV/s ....................................48 
 
Fig 3.2 Cyclic voltammetry of 0.2mM Ru
2
POM in pH=5 buffer solution........................50 
 
Fig 3.3 Cyclic voltammetry of Ruthenium oxidation peaks in pH5 acetate buffer solution 
with 0.2mM Ru
2
POM ........................................................................................................51 
 
Fig 3.4 Cyclic voltammetry of Ir
2
POM in pH=5 Buffer Solution .....................................53 
 
Fig 3.5 Comparison of L-Cysteine oxidation at iridium substituted sandwich POM  
and the precursor Zn2POM, with GC as working electrode, Pt mesh as counter  
electrode and Ag/AgCl as reference electrode...................................................................54 
 
 xv
Fig 3.6 Ascorbic acid oxidation in the presence and absence of iridium substituted 
sandwich type polyoxometalate.........................................................................................55 
 
Fig 3.7 Voltammetric curve showing oxygen reduction reaction at di-iridium substituted 
sandwich POM at a GC electrode in pH5 acetate solution. Pt mesh as counter electrode 
and Ag/AgCl as reference electrode ..................................................................................56 
 
Fig 3.8 Cyclic Voltammogram of Fe
2
POM during the very first scan in 0.2mM  
solution at WE = GC, CE = Pt mesh..................................................................................57 
 
Fig 3.9 Polarization curves for oxygen generation reaction observed in pH = 5 Buffer 
solution at a Glassy Carbon (GC) electrode in the presence of different transition metal 
substituted Polyoxometalates (0.2mM). Counter electrode - Pt mesh and Reference 
electrode-Ag/AgCl (potential values are reported with respect to NHE)???..............58 
 
Fig 3.10 Electrochemical Surface Plasmon Response of Zn
2
POM solution in 0.5 M 
Na
2
SO
4
 corresponding to potential step from -0.2 V (adsorptive) to +0.6 V 
(desorptive)........................................................................................................................60 
 
Fig.3.11 Cyclic voltammogram of 0.2 mM Zn
2
POM in 0.1 M HClO
4 
solution...............61 
 
Fig.3.12 Schematic representation of the breakage of sandwich type clusters and the 
reasons for the changes to the trivacant lacunary sub-units could be any of the following 
three - 1. Very low concentration, 2. Low or high pH condition and 3. Species 
immobilized on electrode surface......................................................................................63 
 
Fig 4.1 pH stability analysis using UV-Vis spectroscopy in 0.5M Na
2
SO
4
  
(pH adjusted by HCl and NaOH).......................................................................................74 
 
Fig 4.2 Cyclic Voltammetric studies depicting pH stability in terms of W redox peaks.  
At pH=9, the ?W? reduction peaks are not well-defined, suggesting the breakdown  
of the compound. WE = Au, CE = Pt mesh, RE = Ag/AgCl, Scan rate 10mV/s. .............75 
 
Fig 4.3 Oxygen Evolution Reaction as a function of pH in 0.2 mM Ru
2
POM. Au as  
WE, Pt Mesh as CE and Ag/AgCl as RE (represented w.r.t RHE) ...................................76 
 
Fig 4.4 Clark Sensor response for the measurement of electrocatalytic generation of 
oxygen from A) Au anode, B) Au anode + 2mM Zn
2
POM, C) Au anode + Ru
2
POM,  
D) Au anode and mono RuPOM........................................................................................79 
 
Fig 4.5 Schematic representation of Rotating Disc Electrode (RRDE) Assembly  
together with the bipotentiostat arrangement. Working electrode = polycrystalline Au 
electrode, Sensing element ? Clark sensor or the Pt ring of the RRDE. Ag wire quasi-
reference electrode (QRFE) was employed. The Counter Electrode (CE) was a high 
surface area platinum mesh. The second figure is the close-up photograph of the  
 xvi
RRDE assembly.................................................................................................................81 
 
Fig 4.6 Electrochemical generation of O
2
 at pH 8 using di-RuPOM catalyst  
characterized using a Rotating Ring Disc Electrode (RRDE). Disc current (upper trace) 
and ring current (lower trace) were acquired simultaneously. The rotation rate was 
1600RPM and the geometrical area of the disc was 0.27 cm
2
. The inset shows an 
expanded view of one set of pulses....................................................................................82 
 
Fig 5.1 Schematic representation of the experimental set-up showing the modified 
SpreetaTM as the working electrode, Pt mesh as counter electrode and Ag/AgCl as 
Reference electrode............................................................................................................94 
 
Fig 5.2 a) Spontaneous adsorption from a solution of 0.2 mM Ru
2
POM in 0.5 M Na
2
SO
4
. 
b) Comparison of SPR response for Ru
2
POM and Zn
2
POM on gold electrode................97 
 
Fig 5.3 ESPR response in 0.1 M HClO
4
 superimposed with the corresponding cyclic 
voltammetry .......................................................................................................................99 
 
Fig 5.4 Electrochemical surface plasmon resonance of 0.2mM Keggin (SiW
12
O
40
4-
)  
in pH4 supporting electrolyte (0.1M NaClO
4
 + HClO
4
) solution. The upper graph  
shows cyclic voltammetry behavior and the bottom graph is the corresponding  
ESPR response...................................................................................................................98 
 
Fig 5.5 ESPR response corresponding to the growth curves shown in Fig 5.6 in 
0.4 mM Zn
2
POM during potential cycling. .....................................................................102 
 
Fig 5.6 Potential Stability testing of the electro-adsorbed film in 0.4 mM Ru
2
POM 
solution in 0.5 M Na
2
SO
4
................................................................................................104 
 
Fig 6.1 Schematic diagram depicting the efficiency losses observed during the  
operation of a typical fuel cell .........................................................................................109 
 
Fig 6.2 Comparison of polarization curves for the oxygen reduction reaction (ORR)  
on several transition metal substituted sandwich type polyoxometalates at a glassy  
carbon electrode (3.0mM Diameter). Supporting electrolyte ? pH = 5 acetate buffer,  
Pt mesh as counter electrode, Ag/AgCl as reference electrode. Scan rate ? 10mV/s......114 
 
Fig 6.3 Thermodynamic correlation between the free energy of M-O bond formation  
and the E
1/2
 for ORR at the transition metal substituted sandwich POM solutions.........115 
 
Fig 6.4 Comparison of polarization curves for ORR at different transition metal 
substituted Wells-Dawson POM solutions at GC (3.0mM Dia) electrode in  
0.1M HClO4 solution (pH=1). Pt mesh as counter electrode and Ag/AgCl as  
reference electrode. (Reported in terms of RHE) Scan rate ? 10mV/s............................116 
 
 xvii
Fig 6.5 Voltammetric scans at an Au electrode immersed in O
2
 saturated 0.1 M HClO
4
. 
POM concentrations were [WPOM] = [FePOM] = [RuPOM] = 0.2 mM and  
[CoPOM] = 0.2 ?M. Scan rate: 10 mV/sec. Inset: Plot of maximum observed ORR 
potential shift, ?E?? vs. ?H?
f
 of the corresponding bulk oxide .......................................117 
 
Fig 6.6 Voltammetric response of a Pd electrode immersed in O
2
 saturated  
0.1 M HClO
4
. POM concentrations were [WPOM] = [FePOM] = [RuPOM] = 0.2 mM 
and [CoPOM] = 0.2 ?M. Scan rate: 10 mV/sec...............................................................121 
 
Fig 6.7 The voltammetric response of a Pt electrode in O
2
 saturated 0.1 M HClO
4
 as a 
function of [CoPOM]. Scan rate: 10 mV/sec.  Inset: plot of ?E?? versus [CoPOM] ......122 
 
Fig 6.8 Voltammetric response of a Pt electrode in O
2
 saturated 0.1 M HClO
4
 containing 
the noted concentrations of PW
12
O
40
3? 
and PCoW
11
O
39
5?
. 
Scan rate: 10 mV/sec. Inset: plot of ?E?? versus [CoPOM]............................................123 
 
Fig 7.1 Cyclic voltammetry comparison of L-cysteine oxidation at V keggin,  
Zn
2
POM and V
2
POM ......................................................................................................134 
 
Fig 7.2 Microdisc method of qualitative analysis of bacteriostatic activity for 
Staphylococcus aureus strain in the presence of silver nanoparticles, V Keggin and 
V sandwich polyoxometalate...........................................................................................136 
 
Fig 7.3 Bacteriostatic activity of the precursor compound Zn
2
POM, in an agar  
medium containing a 10
5
 to 10
7
 CFU/ml of Staphylococcus aureus (S.A).....................137 
 
Fig 7.4 Microdisc method for the study of bacteriostatic effect of Zn
2
POM on  
Salmonella typhimurium (S.T).........................................................................................138 
 
Fig 7.5 Microdisc method to study the bacteriostatic effect of POMs on 10
5
 to 10
7
 
CFU/ml Salmonella typhimurium....................................................................................139 
 
Fig 7.6 Microdisc method elucidating the bacteriostatic property of Zn
2
POM under 
different concentrations ...................................................................................................140 
 
Fig 7.7 Microdisc method to evaluate the bacteriostatic activity of Vanadium POMs  
and precursor in Listeria monocytogenes agar culture plate ...........................................141 
 
Fig 7.8 Time killing assay analysis of Listeria monocytogenes as a function of exposure 
to V Keggin and V sandwich POMs................................................................................142 
 
Fig 7.9 Time killing assay analysis of Salmonella typhimurium as a function of exposure 
to V Keggin and V sandwich POMs................................................................................142 
 
 1
CHAPTER 1 
1. Introduction 
1.1. Motivation for Research 
In today?s world, the energy crisis and global warming are two major 
challenges facing the scientific community. Industries and common households have 
become increasingly dependent on energy for even the simplest processes and oil 
plays the most important role as a driving force for the different economies of the 
modern world. In this scenario, there are a couple of alternatives available to 
scientists ? one to develop more efficient technologies employing oil based systems 
and the other to develop alternative systems. From a chemistry standpoint, all the 
systems ? the industrial machinery, the automobiles and the day today home 
appliances can be envisioned as reactors. In these reactors, the catalyst plays the main 
role in dictating the efficiency and environmental viability. Hence numerous groups 
around the world today are striving tirelessly to develop novel and non-conventional 
catalyst technologies. In this context, we have embarked on a journey to develop 
novel catalysts for alternative energy systems, i.e. polyoxometalates for fuel cell 
applications. 
Polyoxometalates are a unique class of molecular metal-oxide clusters, which 
have interesting properties in terms of catalysis. In this work, an effort has been made 
to design novel polyoxometalate structures to address specific electrocatalytic 
 2
applications. Since polyoxometalates are regarded as models for bulk metal oxide 
catalysts, we have endeavored to replicate the electrocatalytic properties of the latter 
in the molecular scale of polyoxometalates to achieve distinctive advantages in terms 
of ease of synthesis, reliability and energy efficiency, amongst others. 
1.2. Background of Electrocatalysis 
In chemistry, catalysis is defined as the acceleration of rate of chemical 
reactions by contacting with substances (catalysts), which do not themselves show a 
permanent chemical change. These catalysts lower the activation energy of the 
chemical reaction, thus increasing the rate. Depending on the phase of the catalyst 
and substrate, there are two type of catalysis ? heterogeneous (catalyst and substrate 
are in different phases) and homogeneous (both are in the same phase).  In 
electrocatalysis, the catalytic reactions involve reactant and product species 
transferring electrons through an electrolyte ? catalyst interface. Thus electrocatalysis 
applies to the influence of electrode material and the state of electrode surfaces on the 
behavior of chemical-catalyzed steps, which are coupled with electron transfer 
processes. 
The important difference of electrochemistry from other heterogeneous 
catalytic systems is the use of electrode potential as rate-influencing factor. Changes 
in electrode potential, over relatively small ranges can cause very large changes in 
rate for a given reaction, for e.g. about 400 mV can cause three to ten orders of rate 
increase
1
. Such a large change in rate is not possible for reactions occurring in 
solution, since under isothermal conditions, the only available variable will be 
concentration change or pressure change for gases. The only parameter exerting this 
 3
kind of influence on the reaction rate is temperature, which can be considered as an 
analog to electrode potential. A temperature perturbation will change the rates of 
reactions according to their activation energies, however, a variation of ten orders of 
magnitude will require a temperature increase of several hundred degrees (for 
processes at higher end of activation energies) Thus at these higher temperatures, side 
reactions (bond breaking, polymerization) becomes important. 
Further for selectivity, (to differentiate chemical steps of varying activation 
energies), wide variations in temperature are needed, but in electrocatalysis, a mere 
change in potential will accomplish the effect. The selectivity of rates between 
different processes will be related to their different potential sensitivities (Difference 
between their Tafel slopes)  
The advantages of electrocatalysis can be summarized as follows
1
: 
1. Employing the potential variable, there is an ease of control in the rate of 
reaction and novel routes of synthesis of certain organic materials with high 
specificity is achieved 
2. Due to great specificity at high rates, there is higher energy efficiency than 
thermal preparation routes 
3. There is a possibility of direct energy conversion of free energy of combustion 
into dc electricity using fuel cells. This process of energy conversion requires 
the maximum possible reaction rates close to the equilibrium potential for 
anodic and cathodic reactions. 
 
 
 4
1.3. Types of Electrocatalytic mechanism 
The main objective of catalysis is to increase the rate of a reaction and in this 
context; electrocatalysis is aimed towards increasing the rate of an electrochemical 
reaction or in other terms to reduce the overpotential of a given reaction. 
Electrocatalysis can be divided into two types ? Homogeneous electrocatalysis                              
and Heterogeneous electrocatalysis.  
In homogeneous electrocatalysis, the catalytic species is in solution and aids 
in the increased electron transfer between the electrode and substrate. In the case of a 
reduction reaction, electron transfer occurs between the reduced catalyst and the 
substrate, thus regenerating the catalyst. Assuming simple reaction schemes, 
homogeneous electrocatalysis has two mechanistic routes-  
(i) The catalyst behaves purely as an electron mediator between electrode surface 
and the substrate in solution, where a direct redox electron transfer occurs, and 
this type of catalysis is termed ?redox catalysis?.  
(ii) In the second case, an unstable adduct is formed between the reduced catalyst 
and substrate, and this adduct undergoes decomposition after reduction in the 
electrode surface or solution to regenerate the catalyst. This type is termed 
?chemical catalysis?  
In heterogeneous electrocatalysis, the electrocatalyst is immobilized onto the 
electrode surface and aids in facile electron transfer with the substrate. The usual 
routes of catalyst immobilization are electrodeposition, polymer encapsulation, 
physisorption, coating and layer by layer electrostatic methods.  
 
Electrode
Reduced Mediator Oxidized Mediator
O R
 
Fig.1.1. Schematic of mediated electron transfer 
1.4. Polyoxometalate ? Fundamental Introduction 
Polyoxometalates (POMs) are discrete nanometer sized metal oxide clusters, 
which are formed via self assembly of monomeric oxoanions in acidic solutions and 
are very unique in terms of structure and properties. These inorganic metal-oxygen 
cluster anions have been studied from the times of Berzelius, Werner and Pauling 
(1826 ? 1929). However, the first structure to be fully characterized by XRD, in the 
year 1933 by Keggin, and hence these structures, (H
3
PW
12
O
40
.29H
2
O), were named 
after the scientist. The Keggin structures are the best known polyoxometalate 
structures, which can form many different structures and constitute the most stable 
and easily available compounds in this class of POMs.  
Polyoxometalate metal-oxygen clusters contain highly symmetrical 
assemblies of MO
X
 core units, which form a variety of quasi-spherical structures. 
Usually the formulas of these polyanions are as follows: 
 5
 6
 
[M
m
O
y
]
n-
          Isopolyanion  
[X
x
M
m
O
y
]
p-
     Heteropolyanion 
According to Pope
2
, metal M is termed as the ?addenda atom?, usually the transition 
metals from group V and VI. There appears to be a natural limitation dictated by a 
combination of ionic radius, charge and the ability to form d?-p? M-O bonds. There 
seems to be an abundance of W and Mo clusters in literature, while group V 
compounds come second and the Ta & Nb compounds are slowly gaining more 
significance. The atom X, termed as the ?heteroatom? does not have any such 
restrictions and hence almost all the elements in the periodic table can form these 
heteropolyanions.  
In their consideration of polyoxometalate structures, Pope and Muller
3
 have 
suggested that almost all these POM structures can be considered to have originated 
from three highly symmetrical ?parent? polyanions; a. with a tetrahedron (Keggin, T
d
 
symmetry), b. with an octahedron (Anderson Evans anion [XM
12
O
38
], (O
h
) and c. 
with an icosahedron (Dexter Silverton structure, I
h
) as the central polyhedron XO
n
 (n 
= 4, 6 or 12) 
Keggin Polyoxometalate Units 
Keggin structures are spherical molecules (diameter of ~1.2nm), represented by the 
molecular formula, [XW
12
O
40
]
x-8
, where X is the heteroatom, x is the oxidation state 
and M is the addenda atom (W or Mo), with an observed density for 
(H
5
O
2
)
3
.[PW
12
O
40
] as 5.6 g cm
-3
. This well known structure of the POM family has a 
significant number of derivatives and the rich catalytic properties of these have made 
 7
them very relevant in many industrial applications. A central tetrahedron XO
4
 
surrounded by 12 edge and corner-sharing MO
6
 octahedra. Each octahedron is 
arranged in four M
3
O
13
 groups. Each group is formed by three octahedra sharing 
edges and having a common oxygen atom, which is also shared with the central 
tetrahedron XO
4
. There is a total association of 40 close-packed oxygen atoms and 
these can be divided into four types based on their position in the matrix
4
;  
? Twelve M=O,  
? Twelve edge bridging angular M-O-M shared by the octahedral within a 
M
3
O
13
 group 
? Twelve corner-bridging quasi-linear M-O-M connecting two different M
3
O
13
 
groups 
? Four internal X-O-M atoms 
Each of these oxygen atoms can be individually addressed by 
17
O NMR and the 
metal-oxygen bonds provide the signature peaks in the range of 600 to 1100cm
-1
 in 
infrared spectroscopy. Each of the M
3
O
13
 units can be rotated 60? about its 3-fold axis 
and forms geometrical isomers ? the most common is the ?-isomer, and the 
modification of this structure by means of the number of M
3
O
13
 units undergoing 
rotations gives rise to the  new isomers, ?-isomer (one M
3
O
13 
rotated), ? (two M
3
O
13
 
rotated) , ? (three M
3
O
13 
rotated) , and four M
3
O
13 
rotated forms ?-isomer. Lacunary 
Keggin molecules or ?Keggin with holes? can be formed from the removal of one or 
more M atoms. Of the trivacant lacunary structures, if there is a removal of one 
corner-sharing MO
6
 octahedron from each of three bridging M
3
O
13
 triads, it is called 
as A-type and if there is a removal of one entire M
3
O
13
 triad, it is called as B-type
4
. 
 8
Such lacunary species can assemble into larger polyoxometalate structures, by direct 
assembly or by the incorporation of metal ion linkers. A significant amount of 
research has been devoted to the study of trivacant lacunary Keggin moieties because 
they form the largest possible planar minisurface on the plenary XW
12
O
40
n-
 POM. As 
mentioned by Finke
5
, the interaction of trivacant lacunary species with different TM 
atoms can result in structures which could aid in the establishment of POMs as 
models for bulk metal oxide structures. These structures can form Keggin or Wells-
Dawson based sandwich type polyoxometalates. These Keggin units have one 
terminal oxygen atom M=O per each addenda atom, and hence they can participate in 
facile redox reactions. 
Polyoxometalate structures can accommodate transition metals into their 
framework and can form the famous transition metal substituted frameworks similar 
to the organometallic structures. The high negative charges of polyoxometalates are 
known to stabilize these transition metal atoms. Pope et al
6
 were the first group to 
show that it was possible to replace one of the tungsten atoms and the terminal oxo-
group by a transition metal atom cation. These transition metal cations possess five 
bridging oxygen atoms and the sixth coordination site has been shown to be occupied 
by a water molecule.
7
 The sixth coordination site with the labile water molecule is 
considered to serve as an entry to inner-sphere electron transfer pathways, which are 
usually inaccessible in the unsubstituted anions. Though they are completely 
inorganic in nature, these transition metal substituted polyanions have been shown to 
exhibit catalytic chemistry similar to metalloporphyrins
8, 9
. The potential for 
electrocatalytic applications has been very favorable due to the nature of inner-sphere 
 9
as well as outer-sphere electron transfer catalysis. Several transition metal- 
substituted POMs have been synthesized since and the focus towards catalytic 
applications has been driving this field. 
In terms of catalytic applications, transition metal substituted polyanions 
(TMSP) attract a lot of attention, because they can be systematically modified on the 
molecular scale; on the basis of size, shape, charge density, acidity, redox activity, 
stability, solubility, etc 
2, 3, 10
. All these properties contribute favorably towards 
interesting chemical properties, leading to versatile catalytic platforms. Of all the 
commonly occurring TMSP, the class of polyoxotungstates plays a major role in a lot 
of these catalytic interests because they are structurally more stable than molybdates. 
Within the Polyoxotungstates, the Keggin anion and its defect fragments are the basis 
for the structurally diverse molecules
3
. These defect fragments form mono-, di- and 
tri-metal substituted derivatives, and together with Dawson substituted derivatives are 
the most predominant TMSP molecules. The mono substituted Keggin derivatives 
have been considered to be inorganic analogs of porphyrin structures, and they are 
unique in being rigid, hydrolytically stable and thermally robust than their organic 
counterparts
9
. The oxidation resistant tungsten-oxo framework acts as a multidendate 
ligand for transition metals and hence these TMSPs behave similar to organometallic 
clusters
11
, but without the organic framework, which makes them much more stable 
during catalysis. The main problem observed in metal organic clusters during 
reactions is auto-oxidation of the framework leading to structural collapse and loss of 
activity; in this regard the fully inorganic polyanion structure can provide a more 
stable platform. 
 10
 
1.5. Sandwich Type Polyoxometalates 
Among the transition metal substituted polyoxometalates, sandwich type 
structures are very important in the sense that they are similar in construction to 
naturally occurring enzymes and cofactors. Keggin and Wells Dawson structures 
have a predisposition to form these sandwich type structures. Trivacant Keggin 
fragments, often called as the trilacunary species undergo self-assembly to form the 
dimeric polyoxometalate framework - the sandwich structure, also called as Weakley 
or Tourne structures.  These sandwich structures formed by the ?-B Keggin 
fragments can be divided into two subgroups
12
 ?  
1. Central heteroatom X with a lone pair of electrons, forms anions of the formula  
(M(H
2
O))
3
(XW
9
O
33
)
2
]
n-
                   X = As
III
, Sb
III
, Te
IV
, etc 
2. Central heteroatom with no lone pair, forms  
[M
4
(H
2
O)
2
(XW
9
O
34
)
2
]
M- 
                 X = As
V
, Si
IV
, Ga
III
, Zn
II
, etc 
Several other anions are reported, where the positions of the transition metal 
are partially or completely occupied by tungsten atoms
13
, e.g., 
[Zn
3
W(H
2
O)
2
(ZnW
9
O
34
)
2
]
12-
, and in this structure, ZnW
9
O
34
-12
 is the trivacant 
lacunary Keggin structure formed by the removal of ?W
3
O
6
6+
? from the Keggin 
parent ZnW
12
O
40
-6
. The removal of three tungsten atoms is the origin to the 
nomenclature of ?trivacant? lacunary. Since the several metal centers in these 
complexes could be activated independently, there is a need for evaluating the 
potential of these compounds as electrocatalysts. 
 11
The water molecules bonded to the metal atoms can be replaced by other 
organic solvents or sometimes by chloride ions [Cd
4
Cl
2
(AsW
9
O
34
)
2
]
12-
  
14
.  The 
sandwich compounds, as shown in Figure 1.2 are structurally related to the family of 
tetranuclear divalent sandwich complexes [M
II
4
(H
2
O)
2
(PW
9
O
34
)
2
]
10-
 M= Co, Cu, Zn, 
Mn, and were first reported by Weakley, Tourne and co-workers
15
. These sandwich 
structures based on the B-Type trivacant derivatives of the parent Keggin compound 
were later characterized by Finke
5
, Weakley
16
 and Gomez-Garcia et al
17
. Pope and 
co-workers
18
 have reported a mixed valence Mn
II,III
 Keggin sandwich and Craig Hill
19
 
reported the very first sandwich compound with trivalent metal ions in the center, [(n-
C
4
H
9
)
4
N]
6
[Fe
III
4
(H
2
O)
2
(PW
9
O
34
)
2
]. Following these reports, there has been a steady 
increase in the number of transition metal substituted polyoxometalates. Recently 
noble metal substituted sandwich type structures are gaining in importance and 
Neuman et al
20-23
 have paved the way for the development of ruthenium, palladium 
and platinum based sandwich type structures.  
The incorporation of a variety of metals is an important motivation in 
polyoxometalate chemistry and this makes it possible to obtain structures with 
different chemical properties, ultimately providing catalysts with different 
efficiencies.  
 
 
Fig.1. 2. Ball and stick model of sandwich-type polyoxometalate 
 
1.6. Electrochemistry of Polyoxometalates ? Structure/Property Relationships 
The structure of polyoxometalates, are governed by well-known electrostatic 
and radius ratio rules observed for extended ionic lattices. All polyoxometalate 
structures are known to be governed by two general principles, according to Pope
2
,  
1. Each metal atom occupies a MO
x
 co-ordination octahedra or square pyramid, 
where the metal atoms are displaced because of M-O ? bonding, towards the 
vertices 
2. Structures with MO
6
 octahedra with three or more free vertices are not 
observed (according to Lipscomb
24
) 
 12
 13
According to first principles, the metal atoms M are never located at inversion 
centers, since they are displaced towards one side. On the basis of the displacement of 
metal atom within the MO
6
 octahedra, polyoxometalates are divided into two types,  
1. Type I  or Mono-oxo , the metal atom displacement occurs towards one, 
always terminal oxygen atom, called as mono-oxo compound (C
4V
), (these 
compounds can accommodate addenda atoms with d
0
, d
1
 and d
2
 electronic 
configuration) 
2. Type II or cis-dioxo, the metal atom displacement occurs towards two cis, 
usually but not always, terminal oxygen atoms, (these compounds can 
accommodate only d
0
 metals)  
In the case of the type I POM, there is an absence of in-plane ? bonding and 
the LUMO are normally non-bonding and is metal centered (d
xy
 orbital). Thus the 
type I Keggin anions are easily reduced to mixed valence species forming colored 
solutions, commonly called as heteropoly ?blues? and ?browns?, and these reduced 
species maintain the overall architecture of oxidized anions. There is a structural 
affinity to accommodate electrons into the framework and hence the type I Keggin 
atoms have reversible redox properties. From extended Huckel and X? calculations, 
the density of states in the LUMO is supposed to increase, and cause the HOMO-
LUMO gap to decrease, with the aggregation of MO
6
 octahedra. Thus type I 
compounds have predominantly shown ?heteropoly blue? formation in their reduced 
state. Keggin (XM
12
O
40
), Dawson (X
2
M
18
O
62
), hexametallate (M
6
O
19
), decatungstate 
(W
10
O
62
) and related structures belong to this type I category and form the famous 
heteropoly blues, and have been historically employed in analytical applications. 
 14
For the type II POM, the LUMO is strongly antibonding with respect to 
terminal M-O bonds. Thus these structures are reduced irreversibly and there is an 
accompanied structural modification forming complex species, which are yet to be 
identified. Thus type II compounds such as the Anderson structure (XMo
6
O
24
), do not 
form heteropoly blues. The color of the heteropoly blues are the result of the mixed 
valence species formed due to reduction of MO
6
 octahedra, i.e., reduction of the 
addenda atom. If a substituted transition metal is present in the crystal lattice, and 
then there is a possibility of a change in the intensity of color change, pertaining to 
the nature of the transition metal. It has been observed from the redox properties of 
polyoxometalates - the reducibility is governed by the electronic structure of the 
individual MO
6
 octahedra, and this has been suggested to prove that the bonding in 
polyanions is not entirely delocalized as seen in organic aromatic systems. As an 
additional evidence for this difference in delocalization, the possibility of observing 
the diamagnetic susceptibilities of  some select polyanions just by studying the simple 
building blocks such as MoO
4
2-
, WO
4
2-
 and of the heteroatom has been provided 
2, 25
. 
 In the type I form, which is more electrochemically active with reproducible 
redox features, oxidation ability of addenda atoms can be adjusted by the following 
order V(+5) > Mo(+6) > W(+6). In the case of one electron transfer to a mixed 
addenda polyoxometalate species, the electron is localized at the more reducible atom 
at room temperature
26
. The metals incorporated in the heteropolyanion reside in an 
octahedral environment, in which one of the coordination sites is occupied by a 
solvent molecule.  
 
 15
Since these systems are investigated in aqueous solutions, and during 
electrochemical cycling, the following sequential redox states are possible for the 
central metal atom:  metal (III) with aqua ligand reduced to metal (II) with aqua 
ligand, which is oxidized to oxometal (III) then to hydroxo metal (IV) , and finally to 
oxometal (V), for some metals like Re, it is possible to get into (VI) ? (VII) oxidation 
states. The hydroxo metal and oxometal species are important for oxidative synthesis. 
The formal redox potential depends on a lot of factors, of which the nature of addenda 
and hetero-ions are important. For example, the redox potential of incorporated metal 
increases as a function of formal charge on the hetero-ion, A. For a given oxidation 
state, the redox potential increases with the size and decreasing electronegativity of 
X.  The redox potential of Mn (III/II) increases with X, where X= B < Zn < Si < Ge < 
P.  In polar solvents, POMs are deprotonated and could be easily wet-impregnated on 
modified SiO
2
 with methanol as the solvent. Thus POMs can be easily impregnated 
onto traditional catalytic supports. 
1.7. Similarity to Enzymes and Porphyrins 
The development of new and efficient catalysts for multi-electron transfer has 
been one of the reasons for transition metal substituted polyoxometalates (TMSP)
2
 to 
be employed in electrocatalysis. In this context, these TMSPs have been modeled on 
transition metal centered organic complexes similar to enzymes available in nature. A 
number of organometallic compounds have been developed to simulate 
metalloenzymes and POMs have been identified as viable models
27
 (biomimesis) for 
oxidation processes of great biochemical importance.   
 16
Galli et al
27
 have shown that [?-SiVW
11
O
40
]
5-
 and [?-SiMnW
11
(H
2
O)O
39
]
5-
 
can act as biomimetic simulants of laccases, well-known fungal enzymes belonging to 
the class of phenol oxidases. Laccases are multi-copper oxidases, which catalyze the 
one-electron oxidation of phenols in lignins, with a concomitant reduction of 
dioxygen to water. The authors had matched the redox potential of the POMs with 
that of the fungal enzyme in aqueous solution to accurately identify the active species.  
They were successful in performing kinetic studies to determine the mechanism of 
activity in laccases. 
 
1.8. Important Value Adding Properties of Polyoxometalates ? Reasons for 
Choosing this System of Catalysis 
Polyoxometalates exhibit several interesting value adding properties, which 
make them as ideal candidates for catalytic applications, a summary of these 
properties along with some industrial applications are provided below: 
1. Diverse molecular compositions ? Starting from a single structural entity, a 
variety of molecular compositions can be accomplished by very minimal 
changes in transition metal and proton concentration (pH changes) ? Makes 
them suitable as models for fundamental applications. 
2. Reproducible symmetric structures ? All these polyanions are formed by self-
assembly in solution and they exhibit an unique nature of reproducibility in 
terms of symmetry of structures. Symmetry of transition metal placement in 
TMSP has been responsible for a variety of magnetic and electrochemical 
applications. 
 17
3. They are soluble in aqueous and non-aqueous solvents, they maintain their 
structural integrity in solution form; thus making them ideal for both 
homogeneous and heterogeneous catalysis. 
4. Ability to accept multiple electrons without structural modification and very 
good redox reversibility makes them ?tailor-made? for electrocatalysis. The 
possibility of tunable redox potentials by the mere process of replacing the 
substituted transition metals is also an important reason for interest in 
electrocatalysis. 
5. Strong acidity and inert nature of the metal-oxo bond makes them chemically 
and to some extent thermally stable ? important for considerations in catalysis 
6. Relatively low cost and ease of synthesis makes them suitable for 
development of novel catalytic systems for conventional reactions, e.g. 
epoxidation of alkenes
20, 23
. 
7. Polyoxometalates can be easily embedded onto support matrices such as 
carbon, silica and zeolites, thus making them good candidates for 
heterogeneous catalysis. 
Thus, polyoxometalate comprise an elegant class of catalytic metal oxide clusters and 
the study of these structures in the context of electrocatalysis is explored in the 
following chapters. 
 
 
 
 
 18
References 
1. Appleby, A. J., Comprehensive Treatise of Electrochemistry Plenum press: 1983;  
     Vol. 7. 
2. Pope, M. T., Heteropoly and Isopoly Oxometalates. Springer-Verlag: 1983. 
3. Pope, M. T.; Muller, A.,Angew.chem.Int.Ed.Engl. 1991, 30, 34-48. 
4. Kozhevnikov, I., Catalysis by Polyoxometalates. John Wiley and Sons, Ltd: 2002;  
    Vol. 2. 
5. Finke, R. G.; Droege, M.; Hutchinson, J. R.; Gansow, O.,J. Am. Chem.Soc 1981, 
    103, 1587-1589. 
6. Baker, L. C. W.; Baker, V. E. S.; Eriks, K.; Pope, M. T.; Shibatha, M.; Rollins, O. 
    W.; Fang, J. H.; Kohn.L.L,J. Am. Chem.Soc 1966, 88, 2329. 
7. Weakley, T. J. R.,J.Chem.Soc.Dalton Trans 1973, 341. 
8. Katsoulis, D. E.; Pope, M. T.,J.Am.Chem.Soc 1984, 106, 2327. 
9. Hill, C. L.; Brown, R. B.,J. Am. Chem. Soc. 1986, 108, 536. 
10. Keita, B.; Mbomekalle, I. M.; Nadjo, L.; Haut, C.,Electrochemistry 
     Communications 2004, 6, (10), 978-983. 
11. Li-Hua Bi, U. K., Bineta Keita and Louis Nadjo,Dalton Transactions 2004. 
12. Daniel Drewes, M. P. a. B. K.,Journal of Cluster Science 2006, 17, (2), 361-374. 
13. Tourne, C. M.; Tourne, G. F.; Zonnevijlle, F.,J.Chem.Soc.Dalton Trans 1991, (1),  
      143-155. 
14. Hussain F, B. L. H., Rauwald U,  Reicke M, and  Kortz U Polyhedron 2005, 24,  
      847. 
15. Weakley, T. J. R.; Evans, H. T., Jr; Showell, J. S.; Tourne?, G. F.; Tourne?, C. M., 
 19
      J. Chem. Soc., Chem 1973, 139-140. 
16. Weakley, T. J. R.; Finke, R. G.,Inorg. Chem 1990, 29, 1235-1241. 
17. Go?mez-Garc??a, C. J.; Coronado, E.; Go?mez-Romero, P.; Casan?-Pastor, 
      N.,Inorg. Chem 1993, 32, 3378-3381. 
18. Zhang, X. Y.; Jameson, G. B.; O?Connor, C. J.; Pope, M. T.; .Polyhedron 1996,  
     15, 917-922. 
19. Zhang, X.; Chen, Q.; C.Duncan, D.; Lachicotte, R. J.; Hill, C. L.,Inorganic 
     Chemistry 1997, 36, (20), 4381-4386. 
20. Neuman, R.; Khenkin, A. M.,Inorganic Chemistry 1995, 34, 5753-5760. 
21. Neumann, R.; Dahan, M.,Journal of the American Chemical Society 1998, 120,  
     (46), 11969-11976. 
22. Neumann, R.; Dahan, M.,Polyhedron 1998, 17, (20), 3557-3564. 
23. Neumann, R.; Dahan, M.,Nature 1997, 388, (6640), 353-355. 
24. Lipscomb, W.,1965, 4, 132. 
25. Pacault.A; Souchay.P,1949, D377, (Bull.Soc.Chim.France). 
26. Sadakane, M.; Steckhan, E.,Chem. Rev 1998, 98, 219-237. 
27. Galli, C.; Gentili, P.; Pontes, A. S. N.; Gamelas, J. A. F.; Evtuguin, D. V.,New  
      Journal of Chemistry 2007, 31, (8), 1461-1467. 
 
 
 20
CHAPTER 2 
SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF TRANSITION 
METAL SUBSTITUTED SANDWICH-TYPE POLYOXOMETALATES 
 
2.1. Introduction 
There are numerous synthetic procedures directed towards the development of 
novel polyoxometalate structures and the literature has well documented evidence for 
synthesis in aqueous and non-aqueous solvents
1
, incorporation of organometallic 
moeties
2
,  hydrothermal synthesis and to produce giant polyanions using small building 
blocks of pre-formed oxides or synthons
3-5
. Before getting into the basics of synthesis of 
different types of polyoxometalate structures, a brief overview of the structure of these 
compounds is presented.  
The basic heteropolyanion can be viewed as a structure containing numerous 
oxygen atoms (sometimes hydrogen atoms) and fewer atoms of two more elements in 
positive oxidation states. Due to this arrangement, these structures are considered to 
resemble discrete fragments of metal oxide structures possessing unique sizes and 
shapes
6
. Thus they have been employed as molecular models of bulk metal oxide 
catalysis. The first step in the chemical characterization of any compound is to identify its 
precise chemical composition or stoichiometry. In the case of metal oxides, even with 
single crystals or with polycrystalline samples confirmed as single phase, an exact value 
 21
of composition is difficult to obtain. Apart from difficulties and inaccuracies in analytical 
procedures, the phenomenon of non-stoichiometry (e.g. Na
0.7
WO
3
) and the difficulty in 
determining oxygen content changes with synthetic method and this contributes toward 
uncertainty in fundamental properties of some metal oxide systems. The many disputes 
regarding metallic versus non-metallic properties or the existence of magnetic moments 
on transition metal atoms, suggests that different research groups are not studying the 
same compound (particularly differing oxygen content)
7
. In this regard, polyoxometalates 
represent thermodynamically relatively stable structures and hence could account for 
reproducible catalytic properties and better mechanistic understanding. Although, mainly 
in the case of polytungstates, the formation is often under kinetic control rather than 
thermodynamic control
8
. Another characteristic feature of these POMs is that they 
maintain their structural integrity in aqueous and non-aqueous solutions as well as in 
ionic crystals. 
In the polyoxometalate compounds, predominantly W, Mo and V in their highest 
oxidation state act as the ?addenda atoms? (less frequently Ta
5+
, Nb
5+
, Re
7+
, I
7+
) and over 
60 other elements can function as ?heteroatoms?. The addenda atoms have some unique 
characteristics
8
 ?  
1. They can change their co-ordination with oxygen from 4 to 6 as they polymerize 
in solution upon acidification 
2. They have high positive charges and are some of the smaller atoms that fall within 
the radius range for octahedral packing with oxygen atoms. Further the ability to 
form addenda atoms has been known to increase if these atoms form double 
bonds with unshared oxygen of their MO
6 
octahedra, by p? ?d? interaction. The 
formation of a heteropoly complex involves the polymerization of addenda 
polyhedra around a heteroatom 
The reason for the existence of these small discrete heteropoly complexes is due 
to the polymerization of the addenda atoms and the driving force for this process is a 
mechanism involving the attachment of protons to oxygen atoms. Once the structure of 
the heteropoly species is attained (self-assembly), the strong inward polarization of the 
exterior oxygen atoms terminates any further polymerization. Thus the POMs form in 
solutions containing simple oxoanions and the necessary heteroatoms by the self-
assembly process on acidification. The following is an example of such process: 
  
2- 2- + 3-
4 4 12 40 212 WO   + HPO   + 23 H     [PW O ]  + 12H O         (2.1)?
The catalytically active species of Keggin structures are very well known and 
hence they are commercially available, while the rest of the POM structures have to be 
synthesized in-house. The preparation techniques are numerous and are dependent on the 
type of POM to be synthesized. If there is a common trend in the synthetic procedure, it 
should follow the sequence: (i) Precursor compounds of metal ions dissolved in water in 
a ratio corresponding to the final stoichiometry, (ii) Acidification of the solution achieved 
by addition of mineral acids such as HCl, HClO
4
 or HNO
3
. Some groups have employed 
acidic ion-exchange resins (e.g. Amberlyst-15) or electrolysis
6, 9
, (iii) a subsequent heat 
treatment after the addition of heteroatom, either in the presence or absence of air,  
(iv) The heteropoly anions are isolated by addition of an appropriate counter cation ? an 
alkali metal, ammonium or tetra-alkylammonium. Usually, Li and Na salts are more 
water soluble than K, Rb and Cs. Tetrabutylammonium salts are insoluble in water and 
can be recrystallized from acetonitrile or acetone
6
, (v) Recrystallization from the 
 22
 23
appropriate solvent should yield pure compounds to be characterized using XRD, FTIR 
and other analytical techniques such as UV-VIS, Cyclic Voltammetry etc. 
Several other methods of synthesis have been seen in literature, such as electro dialysis, 
hydrothermal and photochemical. 
2.2. Materials and Methods 
2.2.1. Chemicals  
Millipore-Q purified distilled water (18M?) was used throughout. All of the 
chemicals were of high-purity grade and were used as received without further 
purification. The precursors Na
12
[WZn
3
(H
2
O)
2
(ZnW
9
O
34
)
2
].48H
2
O (Zn
2
POM) and cis-
Ru[(CH
3
)
2
SO]
4
Cl
2 
were prepared according to published methods
10, 11
.  
2.2.2. Equipment and Apparatus  
UV-Vis spectra were recorded on a Perkin Elmer spectrometer. All FTIR analysis 
was done on Ru
2
POM sandwich loaded transparent KBr pellets on a Shimadzu 
spectrometer. Single crystal X-ray diffraction studies were performed using G. M. 
Sheldrick, SHELXTL NT/2000, Version 6.1, Bruker AXS, Inc.: Madison, WI 2000. 
2.3. Synthesis of Sandwich Type Polyoxometalates 
2.3.1. Preparation of Zinc Precursor Na
12
[WZn
3
(H
2
O)
2
{ZnW
9
O
34
}
2
]: 
The synthetic procedure developed by Tourne et al
10
, was employed for the 
preparation of the zinc precursor compound, which was the base molecule for the 
subsequent transition metal substituted compounds. A solution of Na
2
WO
4
.2 H
2
O (32 g) 
in water (87.5ml) was treated with nitric acid (14N, 0.5 ml) at 80-85? C with vigorous 
stirring, until the initial precipitate dissolved completely to form a dark yellow solution. 
Then a solution of zinc nitrate (7.5g) in water (25ml) was added very slowly in small 
 24
aliquots (1-2ml), accompanied by continuous stirring and heating at 90-95?C (without 
boiling). During the initial addition of zinc nitrate, a white precipitate was formed, which 
re-dissolved immediately, and after about 2/3 addition, the precipitate started to dissolve 
more slowly and hence the solution was added in a drop-wise manner, so that the mixture 
remained clear till the end. This addition requires about 2-3 hours and the final pH was 
about 7.5. On moderate cooling to about 40 ?C, a first batch of fine needles were 
crystallized. The liquid was evaporated to half the volume and was left unstirred 
overnight. More white needle-like crystals were formed and separated by filtration. The 
resulting cold filtrate was treated with an equal volume of acetone to produce two 
separate layers. The dense lower layer was further diluted and heated to 50 ?C & 
maintained at this temperature overnight to extract more products. The total yield was 
about 75-80%. The entire collected product was subjected to recrystallization from water 
to yield a uniformly hydrated product.  
2.3.2. Preparation of Di-Ruthenium Substituted Sandwich 
Na
14
[Zn
2
Ru
2
(H
2
O)
2
{ZnW
9
O
34
}
2
]: 
Preparation of Ru(DMSO)
4
Cl
2
 as precursor 
 Five grams of RuCl
3
.6H
2
O was mixed with 5 ml of anhydrous DMSO solution 
and the mixture was heated to 80?C for 30 minutes in the presence of an Ar gas blanket. 
After heating under continuous stirring, the mixture was let to cool to produce clear 
orange red crystals of Ru(DMSO)
4
Cl
2
 (yield 50%). The compound was further subjected 
to recrystallization from DMSO solution and tested with FTIR for purity. 
To a solution of Na
12
[WZn
3
(H
2
O)
2
{ZnW
9
O
34
}
2
] (Zn
2
POM precursor, 4.5g in 
10ml water), maintained at 90?C, under a blanket of Ar gas, was added 1.5g of 
Ru(DMSO)
4
Cl
2
. After heating at 90?C for about 18hrs, the mixture was slowly cooled 
and when the temperature is near 45?C, the solution was exposed to atmospheric air and a 
concentrated solution of KCl is added. An orangish brown solution was formed, 
signifying the formation of Ru substituted POM. This step of atmospheric exposure is 
known to be responsible for the formation of Ru
(III)
  from Ru
(II)
 present in the ruthenium 
solvate. On cooling an orangish brown precipitate was obtained (yield 50%). On 
subsequent recrystallization with water, needle-shaped single crystals were obtained. The 
sodium salt Na
14
[Zn
2
Ru
2
(H
2
O)
2
{ZnW
9
O
34
}
2
], was prepared by dissolving the potassium 
salt from the previous step in 0.5 M solution of NaCl. 
Precursor
[WZn
3
(H
2
O)
2
(ZnW
9
O
34
)
2
]
12-
Ruthenium Solvate
Ru[(CH
3
)
2
SO]
4
Cl
2
Molar ratio 
1         :       3
90?C stirred under Ar
For 18 hrs
Na
14
[Ru
2
Zn
2
(H
2
O)
2
(ZnW
9
O
34
)
2
 
Fig. 2.1. Flowchart for the Synthesis of Ruthenium Substituted Polyoxometalate 
 
 25
 
Fig. 2.2. Schematic representation of the central metal belt depicting the lattice position, 
where the transition metal substitution occurs during the formation of sandwich type bi-
Keggin structures. 
2.3.3. Preparation of Ir
(IV)
  substituted Sandwich POMs (Ir
2
POM) 
Noble metal substituted polyoxometalates, such as iridium substituted 
polyoxometalates have been employed as organic oxidation catalysts for the C-H 
activation process
12
. A number of mono-iridium substituted polyoxometalate structures 
exist in literature, Keggin
13
, Wells-Dawson
14
 and organo-polyoxometalate hybrids
15, 16
, 
but to our knowledge, a di-iridium substituted sandwich polyoxometalate has not been 
synthesized. In this context, we have been successful in substituting iridium in the 
sandwich type structure (Weakley structure), first synthesized by Tourne et al
10
. Di-
iridium substituted polyoxometalate Na
12 
[Ir
2
Zn
2
(H
2
O)
2
(ZnW
9
O
34
)
2
] (Ir
2
POM), has been 
synthesized according to the following procedure. 
 26
 27
To a solution of Zn
2
POM (4.5 g in 10ml water) was added 1.5g of anhydrous 
Ir(III) chloride (purchased from Alfa Aesar) and the solution was heated at 90?C for 18 
hours under Ar atmosphere. A dark brown solution was obtained. On subsequent cooling, 
needle shaped crystals of the dark brown coloration was obtained. The crystals were 
subjected to recrystallization with water to obtain uniform water of hydration. 
2.3.4 Preparation of Fe
(III)
 substituted sandwich POMs (Fe
2
POM) 
 The di-substituted iron (III) heteropolyanion was prepared by following 
Tourne?s
10
 method of synthesis. In short, a solution of Fe(NO
3
)
3
.9H
2
O was added in a 
drop wise manner to a stirred solution of precursor Zn
2
POM (molar ratio of Transition 
Metal : Precursor at 3:1) at 90?C. To the resulting light yellow solution was added solid 
sodium acetate and then methanol to obtain a turbid yellow liquid. The sodium salt of 
Fe
2
POM was obtained by crystallization at 0?C, overnight. Despite forming well-defined 
needle like crystals, this compound could not be characterized by single crystal XRD due 
to formation of twinned crystals. (Data could not be solved due to this ?twinning? 
phenomenon. Since these structures have already been solved, we have used vibrational 
spectroscopy and elemental analysis as structural characterization methods. 
2.3.5. Preparation Mn 
(II)
 sandwich POMs 
To a solution of Zn
2
POM precursor, maintained at 80? C and under continuous stirring, 
manganese nitrate solution was added in the molar ratio of 3:1. A yellowish brown 
solution was obtained after heating for 18 hrs. On cooling a powdery precipitate was 
obtained and it was subjected to recrystallization in water. A concentrated solution of 
potassium nitrate was added to crystallize the potassium salt of the POM. A subsequent 
 28
addition of concentrated sodium nitrate solution produced dark brown needle-like crystals 
for single crystal XRD analysis. 
2.4. Structural Characterization 
2.4.1. Elemental Analysis 
Chemical composition measurements were performed on single crystals of sandwich type 
compounds using SEM-EDX analysis and in some cases by ICP-AAS analysis. 
Analytical data for Zn
2
POM (precursor), Ru
2
POM, Ir
2
POM, Fe
2
POM and Mn
2
POM are 
listed in Table 1. The analytical data are consistent with the formulation 
[Zn
2
M
2
{ZnW
9
O
34
}
2
]
n-
 and there is indication towards M(H
2
O) type arrangement for the 
substituted transition metal. (Also confirmed by single crystal XRD studies)  
 
 
 
 
 
 
 
 
 
 
 
 
 
 29
 
Table 2.1. Analytical Data for Na
n
[ZnWM
2
{ZnW
9
O
34
}
2
] 
Complex Na Zn 
M 
Experimental 
(Calculated) 
Na
14
[Ru
III
2
Zn
2
(H
2
O)
2
{ZnW
9
O
34
}
2
]. 
5.8 
(6.17) 
7.92 
(5.01) 
3.79 
(3.88) 
Na
12 
[Ir
IV
2
Zn
2
(H
2
O)
2
{ZnW
9
O
34
}
2
] 
6.6 
(6.4) 
6.3 
(6.18) 
9.11 
(9.09) 
Na
16
[Ni
II
2
Zn
2
(H
2
O)
2
{ZnW
9
O
34
}
2
7.5 
(8.0) 
6.15 
(6.53) 
3.02 
(2.93) 
Na
16
[Mn
II
2
Zn
2
(H
2
O)
2
{ZnW
9
O
34
}
2 
 
7.68 
(8.01) 
6.24 
(6.54) 
3.01 
(2.74) 
Na
14
[Fe
III
2
Zn
2
(H
2
O)
2
{ZnW
9
O
34
}
2
]. 
5.2 
(6.0) 
7.0 
(5.01) 
4.2 
(3.8) 
 
2.4.2 Vibrational spectroscopic studies of selected sandwich polyoxometalates 
Infrared spectroscopy is the first characterization tool in determining the structural 
integrity of the synthesized polyoxometalate. Fig.2.3 shows the FTIR characteristics of 
different transition metal (Ru, Ir, Ni, Mn) substituted sandwich compounds. These studies 
reveal the substituted compounds to be isostructural with the precursor molecule 
Zn
2
POM and clear evidence for the presence of transition metal substitution is not seen as 
the number of heavy tungsten atoms far exceeds the number of TM atoms, since FTIR is 
 30
an averaging technique. Thus the starting material Zn
2
POM and the transition metal 
substituted compounds exhibit similar signature peaks, with very minimal shifts, 
attributed to the transition metal substitution: 
927 cm
-1
      -------  W=O
t
  (Terminal oxo group) 
881 cm
-1  
     -------  W-O-W (Corner sharing WO
6
 octahedra) 
777 cm
-1
     -------  W-O-W (Edge sharing octahedra) 
576 cm
-1 
    -------  W-O-W (Edge sharing mixed or maybe noble metal-oxo  
                                 Stretch 
Further Raman spectroscopic analysis of the single crystal of Zn
2
POM and Ru
2
POM in 
Fig.2.4 have revealed that the common Keggin features were observed and there were 
some other additional peaks, which could be attributed to the sandwich moiety. More 
elaborate work is needed to comprehend the difference between Keggin and sandwich 
type structures. 
1000 900 800 700 600 500
 
Wavenumber / cm
-1
 Zn
2
pom
 Ru
2
pom
 Ir
2
pom
 Fe
2
pom
 Mn
2
pom
 
Fig.2.3 Transmission FTIR data for solid sandwich POMs in a KBr pellet. 
Na
n
[ZnWM
2
{ZnW
9
O
34
}
2
], where M=Ru, Ir, Ni, Mn 
 
 
Fig. 2.4. Raman spectroscopic analysis of single crystals of Zn
2
POM and Ru
2
POM 
 
 31
 32
 
 
2.4.3. UV-Vis Spectroscopy 
300 400 500 600 700
0.0
0.4
0.8
1.2
1.6
 
 
Absor
b
ance (
a
.u)
Wavelength / nm
 Ru
2
POM
 Zn
2
POM
 Ni
2
POM
 Mn
2
POM
 
Fig. 2.5. UV-Vis of sandwich compounds 
In the case of sandwich type polyoxometalates, the UV-Vis spectrum reveals the 
presence of an intense, broad absorption band (below 350nm, not shown for sake of 
clarity) attributed to the usual oxygen to tungsten charge transfer band of Keggin based 
derivatives. For unsubstituted Zn
2
POM precursor and Ni
2
POM, only W-O-W charge 
transfer bands are seen. Ru
2
POM reveals a strong absorption band at 435 nm, which is 
 33
attributed to the oxygen to Ru charge transfer band. Mn
2
POM reveals a broad absorption 
peak centered around 500 nm and from the work done by Tourne
10, 17, 18
, these could 
correspond to a mixture of Mn(II) and Mn(III) species. 
2.5. Single Crystal XRD Analysis 
Zn
2
POM Precursor 
White needle shaped single crystals were obtained by recrystallization from 
aqueous solutions. Careful harvesting of single crystals was performed and mounted onto 
the glass fiber and reproducible results were obtained. 
Ru
2
POM  
Single crystals containing the sodium and zinc salt of the polytungstometalate 
cluster [W
0.92
Ru
1.08
Zn
2
(H
2
O)
2
(ZnW
9
O
34
)
2
]
12? 
(Ru
2
POM) were grown via slow 
evaporation from aqueous solution.  Single crystal X-ray diffraction studies revealed two 
?-B-[ZnW
9
O
34
]
12?
 units linked by a four metal atom belt as shown in Figure 2.6.  Not 
shown in this figure are four [Zn(H
2
O)
5
]
2+
 cations that are bound to the periphery of the 
cluster by the terminal oxo atoms from the polytungstometalate.  The two lobes of the 
cluster are related through a center of inversion.  The four atom belt consists of two zinc 
centers that are five coordinate to the two ?-B-[ZnW
9
O
34
]
12?
 fragments, and in their six 
site by a coordinating water molecule.  W and Ru atoms that are disordered by the 
inversion center occupy the remaining two positions.  In addition, refinement on the 
occupancy of this site reveals a small, but non-negligible, excess of Ru in the belt.  This 
reveals that there must be some compositional variation in the clusters and that some di-
ruthenium clusters exist in the product mixture.  Similar compositional variations were 
observed in [WMZn
2
(H
2
O)
2
(ZnW
9
O
34
)
2
]
12?
 (M = Co, Zn) where slight excesses of Co 
and Zn were found
10
 .   
Crystallographic Data: 
Na
14
{[Zn(H
2
O)
5
]
4
[Zn(H
2
O)]
2
W
0.92
Ru
1.08
(ZnW
9
O
34
)
2
}?28H
2
O : yellow prism, 
dimensions 0.106 x 0.132 x 0.400 mm, triclinic, P1 , Z = 2, a = 12.4771(6), b = 
13.7096(7), c = 16.6901(8) ?, V = 2460.1(4) ?
3 
(T = 193 K), ? = 24.238 cm
-1
, R
1
 = 
0.0497, wR2 = 0.1533.
2
  Bruker APEX CCD diffractometer: ?
max
 = 56.58?, MoK?, ? = 
0.71073 ?, 0.3? ? scans, 24234 reflections measured, 11886 independent reflections all 
of which were included in the refinement.  The data was corrected for Lorentz-
polarization effects and for absorption (SADABS), the structure was solved by direct 
methods, followed by a refinement of F
2
 by full-matrix least-squares with 668 parameters 
and one restraint. Anisotropic displacement parameters were included for all heavy atoms 
and all but four oxygen atoms. 
 34
 
Fig.2.6. ORTEP plot (ellipsoids at 40% probability) of the X-ray structure of 
[Ru
III
2
Zn
2
(H
2
O)
2
(ZnW
9
O
34
)
2
]
14-
. 
 
Ir
2
POM 
The elemental composition performed by EDX and ICP-AAS revealed a 
formulation of Na
12 
[Ir
IV
2
Zn
2
(H
2
O)
2
{ZnW
9
O
34
}
2
] , which corresponds to an oxidation 
state of +4 for iridium. X-ray structure analysis reveals the formation of a sandwich type 
polyoxometalate similar to the Ru
2
POM structure. The ORTEP plot in Fig.2.7, reveals a 
Zn
2
Ir
2
 ring between the two ?-B-[ZnW
9
O
34
]
12? 
units. The large thermal coefficients 
 35
 36
observed for Ir atoms suggest that this position is labile and there seems to be degree of 
disorder between Ir and W atoms, suggesting that there is a mixture of di-substituted and 
mono-substituted single crystal structures. These results suggest that the iridium atom 
substitutes the place of W-atoms in the central metal belt of the precursor molecule 
[WZn
3
(H
2
O)
2
(ZnW
9
O
34
)
2
]
12?
. Unfortunately, it is difficult for the refinement of 
occupancy between Ir and W, as performed in Ru
2
POM, due to the reason that X-ray 
diffraction cannot categorically differentiate between Ir and W, since they are very close 
in the periodic table (negligible differences in atomic weight). Thus the real composition 
of this species has been compared with EDX and ICP-AAS to confirm the stoichiometric 
iridium substitution in the lattice. The exact set of single crystals, which were tested with 
single crystal XRD were subjected to the elemental analysis. 
An important feature about the XRD results was that the iridium substitution 
occurred on the W-atom position of the precursor, and that interatomic distance between 
the Ir atoms was 3.18 ?. Neuman and Tourne
10, 17-19
 have reported that the transition 
metal substitution occurs at the Zn atom position at the farthest ends of the central metal 
belt at an interatomic distance of 5.1 ?. We believe that there has been some discrepancy 
in the crystallographic data analysis and hence we performed some control experiments 
with other transition metal substitutions such as Fe and Mn. We were able to get good 
diffraction quality crystals from Mn
2
POM and crystallographic analysis reveals that the 
transition metal substitution does indeed occur at the nearest ends of the central metal 
belt, at an interatomic distance of 3.18 ?. 
 
Fig.2.7. ORTEP plot of the X-ray structure of Single Crystal X ray structure of Ir
2
POM 
Conclusions 
Synthesis of the transition metal substituted sandwich type polyoxometalates has been 
established and the traditional structural characterization of these compounds has been 
accomplished. In case of a lack of literature record, single crystal XRD analysis was 
performed to prove the uniqueness of crystal structure. 
 
 
 37
 38
References 
1. Errington, R. J., Polyoxometalate Chemistry from Topology via Self-Assembly to 
    Applications. Kluwer: 2007; p 7. 
2. Gouzerh, P.; Proust, A.,Chem. Rev 1998, 98, 77. 
3. A.Muller; F.Peters; M.T.Pope; D.Gatteschi,chem. Rev 1998, 98, 239. 
4. A.Muller; P.Kogerler; C.Kuhlmann,Chem. Commun 1999, 1347. 
5. O.Delgado; A.Dress; A.Muller, Polyoxometalate Chemistry from Topology via Self- 
    Assembly to Applications. 2001; p 69. 
6. Pope, M. T., Heteropoly and Isopoly Oxometalates. Springer-Verlag: 1983. 
7. Cox, P. A., Transition Metal Oxides. Oxford Science Publications: 1992. 
8. C.W.Baker; D.C.Glick,Chem. Rev 1998, 98, 3-49. 
9. G.M.Maksimov,Russ. Chem. Rev 1995, 64, 445. 
10. Tourne, C. M.; Tourne, G. F.; Zonnevijlle, F.,J.Chem.Soc.Dalton Trans 1991, (1),  
     143-155. 
11. Evans, I. P.; Spencer, A.; G, W., J. Chem. Soc., Dalton Trans 1973, 204-209. 
12. Siedle, A. R.; Newmark, R. A.; Brownwensley, K. A.; Skarjune, R. P.; Haddad, L. C.; 
      Hodgson, K. O.; Roe, A. L.,Organometallics 1988, 7, (9), 2078-2079. 
13. Liu, H. Z.; Sun, W. L.; Yue, B.; Jin, S. L.; Deng, J. Q.; Xie, G. Y.,Synthesis and  
     Reactivity in Inorganic and Metal-Organic Chemistry 1997, 27, (4), 551-566. 
14. Sun, W. L.; Yang, F.; Liu, H. Z.; Kong, J. L.; Jin, S. L.; Xie, G. Y.; Deng, J. 
     Q.,Journal of Electroanalytical Chemistry 1998, 451, (1-2), 49-57. 
15. Lin, Y.; Nomiya, K.; Finke, R. G.,Inorganic Chemistry 1993, 32, (26), 6040-6045. 
16. Ishida, K.; Lockledge, S. P.; Klemperer, W. G.,Abstracts of Papers of the American 
 39
     Chemical Society 1993, 206, 347-INOR. 
17. Howard T Evans, C. M. T., Gilbert F Tourne, Timothy J R  
    Weakley,J.Chem.Soc.Dalton Trans 1986, (1), 2699-2705. 
18. Weakley, T. J. R.; Evans, H. T., Jr; Showell, J. S.; Tourne?, G. F.; Tourne?, C. M., J.  
      Chem. Soc., Chem 1973, 139-140. 
19. Neuman, R.; Khenkin, A. M.,Inorganic Chemistry 1995, 34, 5753-5760. 
 
 
 40
CHAPTER 3 
Electrochemical Characterization of Sandwich-Type Transition Metal Substituted 
Polyoxometalate Structures 
3.1. Introduction 
As discussed earlier, transition metal substituted polyoxometalates (TMSP) have 
been employed in many catalytic applications, where, they have replaced bulk metal 
oxides, porphyrins and enzymes. There is a plethora of chemical applications involving 
the oxidation/reduction of the heteroatoms and the addenda ions. Electrochemical 
analysis of these heteropolyanions serves two purposes, one to identify novel redox 
active species and the other to understand the reactivity and mechanism of these species 
as redox reagents and catalysts
1, 2
. The unique properties of TMSPs, such as high stability 
of redox states, tunability of redox potentials, variability of transition metal centers and 
multiple electron transfers
3
 makes them promising candidates for several efficient and 
clean electrocatalytic processes
4
. Several unique applications of polyoxometalates 
(POMs) as electrocatalysts have appeared in the literature and a couple of good reviews
3, 
4
 illustrate the state of the art in today?s scientific world. Nadjo
5-11
 and Hill
8, 10, 12, 13
 
groups have made several noteworthy contributions to this field. Of all the POMs 
available, Keggin and Wells Dawson based structures form the bulk of 
electrocatalytically active species. Literature review shows that polyoxometalates are 
active in both electrocatalytic oxidation and reduction reactions, provided the right POM 
 41
and conditions are chosen
4
.  This chapter deals with a short review of the electrochemical 
properties of TMSPs and basic electrochemical characterization of the sandwich type 
polyoxometalates comprising the two Keggin units connected by a belt of four transition 
metals in the center. Na
n
(M
2
Zn
2
[ZnW
9
O
34
]
2
), where M is the substituted transition metal 
atom. Electrochemical characterization of sandwich POMs has been performed to 
identify and understand the nature of the electroactive species to shed some light on the 
electrocatalytic properties of these structures. 
3.2. Electrochemical Properties of Transition Metal Substituted Polyoxometalates ? 
Literature Review 
As described in the first chapter, polyoxometalates can be classified into two 
groups, ?mono-oxo? (type I) and ?cis-oxo? (type II), based on their redox properties
1
. 
This classification is predominantly based on the number of terminal oxygen atoms 
attached to the addenda atom, tungsten or molybdenum. It has been shown that only type-
I POMs are easily reduced and hence can form reversible mixed valence species, without 
any major changes in the structure. These redox properties are further enhanced when 
different transition metals are substituted in these frameworks. Depending on the final 
application, POMs were chosen with favorable thermodynamics and kinetics for the 
redox transformations. In this regard, POMs have been employed as oxidation catalysts 
for clean synthesis of organic substances in the industrial world. 
In TMSP, the presence of the sixth coordinated water molecule on the transition 
metal contributes to the possible entry point for inner-sphere electron transfer
8
. Further, 
the inorganic nature of the cluster combined with their resistance to conventional 
degradation processes, has been attractive properties for electrocatalysis. The potential 
 42
for both inner-sphere and outer sphere electron transfer phenomena has contributed to the 
substantial volume of research in the literature of electrocatalysis of POMs. Since, 
efficient electrocatalysis of multiple electron transfer reactions is important in many 
industrial reactions, the stable and reversible multiple electron transfer properties of 
POMs are particularly conducive. Usually the POMs are employed to replace 
conventional transition metal complexes (capable of only single electron transfer), which 
are the metalloporphyrins and organometallic complexes, and these compounds are 
frequently susceptible to degradation (oxidation) by reactive intermediates formed during 
the process (ref). When there is a possibility of multiple electron transfer (up to five 
electrons) as in POM, the formation of highly reactive radical intermediates is avoided, 
given that numerous oxygen ligands of POMs are inherently inert towards oxidizing 
environments. 
Large number of polyoxometalates undergoes a series of reversible one or two 
electron reductions. These reactions are typically substantiated using cyclic voltammetry 
which provides electrochemical fingerprints for these structures. It has been found that, if 
the substituted transition metal atoms are not reduced, then the E
1/2
 or redox potential of 
TMSPs will be very similar to their precursors. In these cases, the reduction process and 
the structure of the polyanions are considered to be the same as the precursor molecules. 
If the substituted transition metal atoms are reduced, then the reduction process and the 
number of reduction waves will undergo changes corresponding to the nature of the 
substituted metal. 
 Another desirable property in POM based electrocatalysis is the ability to fine-
tune the redox potential of transition metal center over a wide range by altering the 
 43
identity of the heteroatom located in the center of the tungsten-oxide cage
14-18
. This 
property has been described as ?tuning by synthesis?. This particular aspect of POM 
electrocatalysis has been employed as a diagnostic tool to study the mechanisms of 
electron transfer reactions during the catalysis.  
General Understanding of POM electrochemistry 
In the case of POMs, the unique symmetry in structures contributes to the 
equivalence of several metal centers of a given framework. However, during electron 
transfer in a Keggin molecule, such as PW
12
O
40
3-
, a step-wise reduction of the tungsten 
centers takes place
8
. EPR experiments have revealed that an added electron undergoes 
delocalization over all of the twelve tungsten centers and this has been observed has the 
general trend for all unsubstituted and highly symmetrical Keggin anions. Introduction of 
lower symmetry by substitution of transition metal atoms creates substantial 
modifications in this degree of valence trapping.  
POMs have been used as model systems to mimic the electronic properties of 
metal oxides at the molecular level (ref). In general, the fully oxidized anions are known 
to have a simple electronic structure with a set of doubly occupied orbital?s, which are 
centered at the oxo-ligands and are well separated from the occupied orbitals
19
 (centered 
at metal atoms). Fully oxidized POMs are easily reduced in solution and this is achieved 
because the lowest unoccupied molecular orbital (LUMO) of POM, which is a symmetry-
adapted non-bonding orbital (d
xy
-like), is always low in energy
20
.  In the case of one of 
the addenda atoms, W
VI 
or Mo
VI
 being replaced by another early transition metal such as 
V
V
, Nb
V
 or Ti
IV
, the redox properties of these compounds have been well understood in 
terms of both theory
21
 and experiment
22
. However, the incorporation of a transition metal 
 44
(TM) in a lower oxidation state such as Fe
III
, Co
II
, Ni
II
 etc onto the POM framework is 
not well understood and hence several hypotheses are prevalent and more experimental 
investigations are needed. In the case of TM substitution in Keggin and Wells-Dawson 
derived sandwich-type polyoxometalates, which have been employed in numerous 
applications in catalysis, medicine, pharmaceutical etc, the interaction of the substituted 
transition metal with the POM structure and its influence on the electron transfer 
behavior is also very interesting.  
3.3. Materials and Methods 
3.3.1. Polyoxometalates and Reagents 
All sandwich type polyoxometalate structures were synthesized as described in 
the previous chapter. BAS glassy carbon electrode or BAS gold disc electrode were 
employed as working electrodes. Supporting electrolyte solutions were unbuffered 0.5M 
Na
2
SO
4 
(pH~ 6) and buffered acetate solutions at pH 5. 
3.3.2. Electrochemical Measurement Set-up 
The solutions were thoroughly deaerated with pure Ar gas for 20 minutes and 
kept under positive pressure of this gas during all electrochemical experiments. All cyclic 
voltammetric measurements were performed on a 20ml glass vial with a home made 
mounting head to place the electrodes at constant inter-electrode distances. The 
electrochemical set-up was a three electrode arrangement and driven by a BAS-Epsilon 
workstation. All experiments were performed with Ag/AgCl/KCl (satd.) reference 
electrode, unless otherwise stated. In order to prevent contamination by polyoxometalate, 
a salt bridge filled with 0.1M NaClO
4
 was employed. The counter electrode was platinum 
gauze of a large surface area. The experiments were performed at laboratory temperature.  
 45
3.3.3. Cyclic Voltammetry of Transition Metal Substituted Sandwich type 
Polyoxometalate  
Glassy carbon was employed as the working electrode, Pt mesh as counter 
electrode and Ag/AgCl as reference electrode. In order to prevent contamination, a salt 
bridge containing the pure electrolyte was used for housing the reference electrode and 
was placed very close to the working electrode. GC electrode was subjected to polishing 
with alumina nanoparticles and then tested for consistent electrochemical characteristics 
in 0.1M HClO
4
.  The concentration of Zn
2
POM and Ru
2
POM was maintained at 0.2mM 
and the corresponding cyclic voltammetric measurements were made, first by cycling 
from negative potential to positive. An initial scan was performed at 100mV/s (not 
always shown) and then exploratory scans were performed at slow scan rates of 10mV/s, 
with the potential window being expanded as necessary. 
3.4. Results and Discussion: 
3.4.1. Electrochemical Behavior of Sandwich POM Structures 
In our attempt to understand the electrochemical behavior of the sandwich type 
polyoxometalates, it will be an imperative step to discuss the well-established 
electrochemistry of Keggin type polyoxometalates, for example SiW
12
O
40
4-
.  In 1 M HCl 
solution, there are five reduction peaks with the approximate electron ratios of 1:1:2:8:12. 
The characteristic signature of Keggin molecule has been known to be comprised of the 
first three redox peaks, which are well-defined and reversible in nature. The origin of 
these redox features have been attributed to the addition and removal of electrons from 
primarily non-bonding orbital?s within the four tungsten oxide triads (W
3
O
13
) within the 
Keggin polyanion. The fourth and fifth reduction waves are accompanied by chemical 
reactions of the cluster contributing to adsorption of tungsten compounds onto the 
electrode surface. The first two one electron redox peaks were essentially unaltered under 
condition of pH < 5. The third wave, consisting of two electron reduction has been shown 
to move to more negative potentials by about 59mV/pH unit. The three redox waves can 
be described by the following equations, 
4- -5-
12 40 12 40
5- - 6-
12 40 12 40
6- - + 6-
12 40 2 12 40
           SiW O  + e    SiW O               (3.1)
          SiW O  + e     SiW O               (3.2)
 SiW O  + 2e + 2H   H SiW O         (3.3)
                               
?
?
?
 1 < pH < 5
 
For the case of the monolacunary Keggin atom, SiW
11
O
39
8-
, with one tungsten 
cation missing, the electrochemical response comprises of two waves caused due to two 
electron transfer reactions and the formal potentials have been shown to be significantly 
more negative than the parent Keggin polyanion. 
In our study, the electrochemical behavior of Na
n
(M
2
Zn
2
[ZnW
9
O
34
]
2
), a sandwich 
type polyoxometalate was studied by employing cyclic voltammetry to identify the 
electroactive species in solution and to evaluate the feasibility for electrocatalysis. 
Traditionally, the stability of these sandwich type POM structures has been evaluated by 
monitoring the UV-VIS spectra under different pH conditions to determine the ideal 
medium for electrochemical analysis. The reproducibility of the UV-VIS response, in 
terms of absorption spectra and wavelength positions have been known to provide the 
stability conditions between pH 4 and 9 and this has been shown to be a good starting 
point for reliable cyclic voltammetry. Outside of this range, the cyclic voltammograms of 
these anions were ill-defined and the heteropolyanions were unstable with respect to 
 46
 47
degradation. In this chapter, the electrochemical behavior of transition metal substituted 
sandwich type structures (Zn
2
POM, Fe
2
POM, Ni
2
POM, Cu
2
POM, V
2
POM, Mn
2
POM, 
Ir
2
POM and Ru
2
POM) were studied to understand the effect of transition metals as 
heteroatoms in the sandwich polyoxometalate framework. The series of transition metal 
substituted sandwich compounds were expected to exhibit not only the step-wise 
reduction processes of the addenda atom (W), but also some of the redox features of the 
substituted transition metal atoms. In this regard, the electrochemical behavior of these 
sandwich POM structures could have some similarity to the transition metal substituted 
Keggin type structures, as described by the groups of Pope 
1, 23-25
, Keita 
4, 26-28
 and 
others
29-32
. In some cases, for the sake of simplicity and clarity, the electrochemical 
behavior of the respective W atoms and the substituted central metal atoms are described 
separately in the following sections. We have understood from literature that transition 
metal substitution causes a change in the position of tungsten redox peaks; there is a 
common observation of these peaks to be shifted to even more cathodic potentials in 
Keggin structures. 
As we have seen before, the sandwich type structures, Na
n
[Zn
2
M
2
{ZnW
9
O
34
}
2
] 
are formed by the assembly of two trivacant Keggin molecules onto a central metal belt 
comprising four transition metal atoms in a rhombus-like arrangement. Since the 
trivacant lacunary B-ZnW
9
O
34
12-
 molecules are not known to exist in the isolated solid 
state, 
1
 (they are difficult to isolate from solution), all electrochemical measurements have 
been compared with the literature data for trivacant lacunary fragments of 
phosphotungstate (?-[PW
9
0
34
]
n-
). The effect of substitution of each transition metal (V, 
Mn, Fe, Ni, Cu, Ir, Ru) on the electrochemical behavior of the sandwich compounds have 
been studied and the results have been summarized in the following section. 
Zn
2
POM (Precursor) Na
12
[WZn
3
(H
2
O)
2
{ZnW
9
O
34
}
2
]  
The sodium salt of the Zn
2
POM precursor in a pH 5 buffer solution has been 
studied by cyclic voltammetry and the predominant redox features obtained during a 
steady state condition has been shown in Fig.3.1. No other peaks are visible in the higher 
anodic potential region, signifying the absence of Zn redox features. This voltammogram 
shows the fingerprint features analogous to those observed for [PW
9
O
34
]
n- 
based 
sandwich type structures
33, 34
.  
 
Fig.3. 1 Cyclic Voltammogram of 0.2mM Zn
2
POM in pH=5 buffer solution, with Glassy 
Carbon as Working Electrode (W.E), Pt mesh as Counter Electrode (C.E) and Ag/AgCl 
as Reference Electrode (R.E), at a scan rate of 10mV/s. 
From Fig.3.1, two sets of redox peaks are clearly visible at the formal potentials, 
E
1/2
 of -0.23V and -0.4 V. The difference between E
pa
 (I,II) and E
pc
 (I?,II?) was generally 
in the range of 0.059 to 0.074 V, where E
pa
 and E
pc
 are the anodic and cathodic peak 
 48
 49
potentials. Both the peak potentials were independent of potential scan rates in the range 
from 10 to 100mV/s. These results have suggested that the electrode reactions of the 
Zn
2
POM precursor compound are reversible or quasi-reversible. From the previous 
studies
29-31
 on the phosphotungstate sandwich compounds, we can correlate the two redox 
peaks observed in Fig.3.1 to the tungsten (VI) redox peaks. These peaks are pH 
dependent and they exhibit a reversible nature until a pH of 9 in the alkaline direction. 
After this, the redox behavior becomes ill-defined in high pH conditions, and hence no 
further studies were performed in this direction.  
Further, each redox peak current has been found to be proportional to the two 
thirds power of the scan rate and this signifies that the redox features are surface-active in 
nature
35
. It has been well-established that the two-thirds power scan rate dependence of 
peak current corresponds to a two-dimensional phase transition or a nucleation and two-
dimensional growth mechanism.
36, 37
  
Ru
2
POM sandwich, Na
14
[Ru
III
2
Zn
2
(H
2
O)
2
{ZnW
9
O
34
}
2
] 
The diruthenium substituted sandwich type polyoxometalate has been synthesized 
by the reaction of cis-Ru(dmso)
4
Cl
2
 with the Zn
2
POM precursor, as described in the 
second chapter. In earlier studies, Keita et al
27, 33, 38
 have prepared several Ru supported 
POMs such as [Ru(dmso)
3
(H
2
O)-XW
11
O
39
]
n-
, where X=Si or Ge, using similar organic 
ruthenium solvates as precursors together with lacunary species of the kind [A-
XW
9
O
34
]
10-
, in aqueous acidic media. The presence of the organic component of the Ru 
precursor in the polyoxometalate framework was a common trend in all of the studies 
conducted by this group. In our compound, detailed elemental analysis and single crystal 
studies have revealed the absence of the DMSO fragment. Thus, the diruthenium 
substituted sandwich compound was completely inorganic in its structural composition.  
 The steady state voltammetry of 0.2mM Ru
2
POM at a glassy carbon electrode 
reveals some interesting differences when compared to the Zn
2
POM characteristics. As 
shown in Fig.3.2, a pair of redox couples at the most negative potentials corresponding to 
W(VI) were observed at formal potentials E
1/2 
at -0.30 V and -0.42 V. The slight negative 
shift of these redox peaks in comparison to those of the parent Zn
2
POM, has been 
observed in literature and is a consequence of transition metal substitution
29, 31
. There is 
an additional redox couple at E
1/2 
of -0.15 V could be attributed to the redox state of the 
substituted Ru metal centers. From Fig.3.2, we can identify the redox features of W cage 
and the first redox couple of ruthenium metal centers. A small ill-defined shoulder of a 
redox peak is visible at the E
1/2
 of +0.12 V.  
 
Fig.3. 2 Cyclic voltammetry of 0.2mM Ru
2
POM in pH=5 buffer solution 
 50
From comparison with previous ruthenium substituted polyoxometalate structures 
containing Ru(III) centers by Pope
39
, Keita
27, 33, 38
 and Hill, the redox features correspond 
to the stepwise reduction of the ruthenium metal centers. Further analysis of the cyclic 
voltammogram in the positive direction as shown in Fig.3.3 reveals the presence of one 
Ru oxidation peak(E
1/2 
= 0.95V), one irreversible oxidative peak (1.2V) and one 
irreversible reduction peak at 0.7V.  
 
Fig.3.3. Cyclic voltammetry of Ruthenium oxidation peaks in pH5 acetate buffer solution 
with 0.2mM Ru
2
POM 
Ir
2
POM sandwich  
Electrochemical investigations have been performed to establish the iridium 
substitution in the framework. The electrocatalytic effect of this compound has been 
tested for oxidation and reduction reactions, i.e., oxidation of water (oxygen generation 
 51
 52
reaction), oxidation of L-cysteine & ascorbic acid and oxygen reduction were studied. 
According to our knowledge, this is the very first time that a bifunctional electrocatalytic 
nature has been experimentally tested on a di-iridium substituted sandwich 
polyoxometalate. It has been well established in literature that the mono-substituted 
Iridium Keggin and Wells-Dawson structures are very good electrocatalysts for nitrite 
reduction in acidic medium
40-42
.  
In this work we have concentrated on the electrocatalytic applications in
near neutral conditions. Our interest in novel applications of transition metal substituted 
polyoxometalates had prompted us to explore amino acid oxidation, which is very 
important in neutral medium and hence, can be employed in biosensing systems. Keita et 
al
11, 43
 have developed Vanadium substituted Wells-Dawson structures to be efficient 
electrocatalysts for L-cysteine oxidation. Of all the transition metal compounds tested, 
the vanadium based system was the most efficient in their studies. Ascorbic acid 
oxidation is also an important reaction occurring at near neutral conditions and several 
groups have explored polyoxometalate based electrocatalysis
31, 44
 for this reaction.  
 
Fig.3. 4 Cyclic Voltammetry of Ir
2
POM in pH=5 Buffer Solution  
 
In the case of Ir
2
POM (Fig.3.4), the cage features are very similar to the Zn
2
POM 
redox waves and similarities with Ru
2
POM can be made. A slight shift in the Ru
2
POM 
cage feature can be attributed to the difference in their valencies (Ru
(III)
 and Ir
(IV)
). 
L-cysteine oxidation occurs at very high overpotentials in glassy carbon electrode 
at neutral pH conditions
43, 45
, despite this fact, L-cysteine sensing plays an important role 
as a biological marker molecule
11
. In this regard, the stability of sandwich type structures 
in neutral conditions is an important advantage and hence the application of Ir
2
POM has 
been explored. Fig.3.5 reveals the linear scan voltammogram of 0.2mM Zn
2
POM and 
Ir
2
POM solutions, in the presence of L-cysteine (0.05mM).  
 53
 
Fig..3.5. Comparison of L-Cysteine (10 ?M) oxidation at iridium substituted sandwich 
POM and the precursor Zn
2
POM, with GC as working electrode, Pt mesh as counter 
electrode and Ag/AgCl as reference electrode.  
The precursor Zn
2
POM shows an oxidation peak, which begins at about 0.80V. 
Ir
2
POM on the other hand exhibits an oxidation peak very early at about 0.45V. Since the 
cysteine oxidation peak begins just after the iridium oxidation peak, which is centered on 
0.01V under these pH conditions, the transition metal plays an important role in this 
oxidation reaction. It has been shown in literature
40, 42
 that Ir substituted 
polyoxometalates exhibit some unique reductive electrocatalytic properties. In this work, 
the presence of a di-iridium substitution has provided improved oxidative capability.  
 54
0.5 1.0
 
Potential / V vs RHE
 Pure GC
 Ir
2
POM
2 ?A
 
Fig.3.6. Ascorbic acid (10 ?M) oxidation in the presence and absence of iridium 
substituted sandwich type polyoxometalate. 
In a similar direction, the performance of di-iridium for ascorbic acid oxidation 
has also been evaluated. As shown in Fig.3.6, the oxidation reaction of ascorbic acid 
occurs about 0.1V to the left of the pure glassy carbon electrode. This behavior is unique 
to the di-iridium substituted species and not shown in the Zn
2
POM precursor molecule. 
Further explorations are needed to identify the mechanism of oxidation of organic 
molecules to understand the role played by the iridium substituted onto the Zn
2
POM. In 
this regard, iridium substituted sandwich can be considered as similar to vanadium 
substituted sandwich compounds as described by Keita et al
5, 46, 47
.  
Further studies on the reduction side of Ir
2
POM voltammetry, revealed an 
interesting behavior in the presence of oxygen in solution. It has been well established 
 55
that polyoxometalates catalyze the reduction of oxygen. In our case, as shown in Fig. 3.7, 
in a pH=5 acetate buffer, there is a steady increase in current, exactly after the iridium 
reduction peak. This has been attributed to oxygen reduction by the iridium metal center.  
 
Fig.3.7. Voltammetric curve showing oxygen reduction reaction at di-iridium substituted 
sandwich POM at a GC electrode in pH5 acetate solution. Pt mesh as counter electrode 
and Ag/AgCl as reference electrode. 
The electrolyte solution was bubbled with oxygen for more than 30 minutes and the 
oxygen concentration was considered to be constant and equivalent to saturated values at 
the given atmospheric conditions. 
Fe
2
POM Sandwich {Na
14
[Fe
III
2
Zn
2
(H
2
O)
2
{ZnW
9
O
34
}
2
]} 
Iron substituted polyoxometalates have been extensively studied due to their 
unique electrocatalytic property directed towards H
2
O
2
 reduction, nitrate reduction etc.  
 56
 
Fig.3. 8 Cyclic Voltammogram of Fe
2
POM during the very first scan in 0.2mM solution 
at WE = GC, CE = Pt mesh 
The cyclic voltammogram of Fe
2
POM solution as shown in Fig.3.8 exhibits the 
usual tungsten (VI) reduction peak at -0.30 V and an additional peak at +0.09V in the 
anodic region. From previous studies, this peak could be assigned to Fe
3+/2+
 redox peak. 
The poorly-defined redox peaks observed during the initial cycling is a consistent trend in 
these sandwich compounds, and is also observed in the Zn
2
POM. Electrocatalytic activity 
of Fe
2
POM for oxygen reduction will be discussed in subsequent chapters. 
 
3.5. Electrocatalytic oxygen generation at transition metal substituted sandwich type 
polyoxometalates 
In order to evaluate the electrocatalytic property of these TMSP sandwich 
 57
compounds, oxygen generation through water oxidation was selected as the model 
reaction. Glassy carbon was chosen and a near neutral pH (~5) was chosen for study as 
shown in Fig.3.9. This qualitative cyclic voltammetry establishes the fact that di-
ruthenium substituted sandwich compound is the most active complex in glassy carbon 
electrode. Following Tourne?s method
48
, Fe, Mn and Ni substituted sandwich type POMs 
were also synthesized.  
As observed from the polarization curve in Fig.3.9, the Ru
2
POM solution exhibits 
the very best performance followed by Ni, Mn and Fe. For the Ru
2
POM, the onset of 
oxygen evolution occurs at 0.94 V, which matches with the thermodynamic potential 
corresponding to oxygen evolution at pH=5 solution. 
 
Fig.3.9 Polarization curves for oxygen generation reaction observed in pH = 5 Buffer 
solution at a Glassy Carbon (GC) electrode in the presence of different transition metal 
substituted polyoxometalates (0.2mM). Counter electrode - Pt mesh and Reference 
 58
 59
electrode-Ag/AgCl (potential values are reported with respect to NHE) 
 3.6. Stability of transition metal substituted sandwich polyoxometalate thin films 
Films of sandwich type polyoxometalates are formed as a result of delicate self 
assembly under very precise conditions of pH and concentration. In this regard, we 
believe that there is a dynamic equilibrium exists between the integral sandwich 
compounds and their precursor molecules in solution during potential cycling also. There 
could also be an effect of localized pH variation in a non-buffered electrolyte. There is a 
possibility of derivatization of the electrode surface during the initial cycling procedure, 
in the case of Zn2POM. Keita et al
33, 34
 have shown that extending the potential window 
of trivacant lacunary phosphotungstate species to more cathodic regions (> -0.7 V) results 
in the deposition of a film and this film has been described to render the voltammogram 
featureless. Keeping these concepts in mind, we have explored the behavior of Zn
2
POM 
sandwich as unique structures. As an example for adsorption on the surface of electrodes, 
we have performed electrochemical surface plasmon resonance studies of Zn
2
POM. An 
elaborate exploratory study has been performed in the chapter 5. Here we present a 
representative plot of the behavior of Zn
2
POM solution in the presence of a potential 
step. The Fig.3.10 show the response of ESPR, an interface exploratory technique, which 
shows changes in optical properties with increase in interfacial activity. 
0 200 400 600 800 1000 1200 1400
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
??
SP
R
Time / Sec
-0.2V
+0.6 V
-0.2 V
+0.6 V
 
Fig.3.10 Electrochemical Surface Plasmon Response of Zn
2
POM solution in 0.5 M 
Na
2
SO
4
 corresponding to potential step from -0.2 V (adsorptive) to +0.6 V (desorptive) 
It has been shown that at very high pH conditions, the whole polyanion undergoes 
decomposition via the OH
-
 ion attack. From UV-Vis studies of these compounds, it has 
been established that the sandwich POMs have a stability window from pH 4 to pH 9
33
. 
In order to understand the decomposition of sandwich type structures, we have 
deliberately acidified a solution of Zn
2
POM and subjected it to electrochemical 
characterization. A single investigative cyclic voltammogram was recorded for this 
Zn
2
POM precursor in an acidic medium (ca. 0.1M HClO
4
), as shown in Fig.3.11. The 
three redox couples in 0.1M HClO
4
 solution can be labeled by a preliminary comparison 
with the redox peaks of a monolacunary Keggin structure
3
 (ZnW
11
O
39
10-
). The first two 
reduction peaks (I? and II?) could be equated to the two one-electron waves seen in 
Keggin structures and the third peak (III?) could be equated to the two electron reduction 
 60
accompanied by proton addition. The number of electrons transferred have been 
calculated using constant potential electrolysis and by comparison of the magnitude of 
current in the case of one electron reduction of potassium ferricyanide solution at a glassy 
carbon electrode with the same geometric area. The position of the redox peaks in 
Fig.3.11 reveals a similarity to the characteristic tungsten redox features of the 
corresponding zinco-tungstate monolacunary Keggin molecule, ZnW
11
O
39
10-
. This 
observation, coupled with the fact that the stability of these sandwich compounds reside 
in the range of pH 4 to 9, points to the phenomenon of the fragmentation of the sandwich 
into lacunary Keggin-like structures.   
 
Fig.3. 11 Cyclic voltammogram of 0.2 mM Zn
2
POM in 0.1 M HClO
4 
solution 
At this juncture, it will be interesting to discuss the nature of the fragmentation of 
clusters in the acidic medium. As mentioned earlier, these sandwich compounds are made 
 61
 62
of two trivacant lacunary Keggin units attached by a central metal belt comprising four 
transition metals. Hence a natural candidate for the fragment will be the trivacant 
lacunary structure, ZnW
9
O
34
12-
. These trivacant lacunary species have been described 
previously, but its structural characterization had always been based on indirect methods, 
since these species are known to exist in solution at a relatively low half-life during the 
pH dependent equilibria with the monolacunary species (ca. PW
11
O
39
7-
) and other units. 
Finke et al
49
, were the first group to identify conditions to stabilize these trivacant Keggin 
units using an M
2+
 substitution, by titration with Co
2+
. When these structures were 
subjected to crystallographic investigations, it was found that the PW
9
M
2
O
34
5-
 species 
had dimerized to form a saturated heteropolytungstate P
2
W
18
M
4
(H
2
O)
2
O
68
10-
. This is the 
tetra metal substituted version of the sandwich complex under investigation in our study 
[(M
2
Zn
2
[ZnW
9
O
34
]
2
)
n-
]. Finke?s group was successful in developing an alternative 
synthetic route for these sandwich compounds. This finding presents strong evidence 
towards the presence of a bimetal substituted trivacant lacunary species in solution during 
the decomposition or cluster breakage as shown in Fig.3.12. 
[(M
2
Zn
2
[ZnW
9
O
34
]
2
)
n-
]
ZnW
9
O
34
12-
M
2
[ZnW
9
O
34
]
M1
M1?
M2M2?
M2M2?
M1
M1?
M2
M1
M1?
M2?
 
Fig.3.12 Schematic representation of the fragmentation of sandwich type clusters and the 
reasons for the changes to the trivacant lacunary sub-units could be any of the following 
three: 1. Very low concentration, 2. Low or high pH condition and 3. Species 
immobilized on electrode surface. 
A comparison of the behavior of Zn
2
POM in acidic medium (Fig.3.11) and at 
pH=5 (Fig.3.2), reveals another important feature that the redox peaks in pH=5 are not 
well-defined and they do not exhibit diffusion limitation, unlike the peaks observed under 
acidic conditions. This behavior shows that these redox peaks could be the result of 
surface active species, and subsequent studies have been performed to elucidate this 
observation. The presence of two trivacant lacunary Keggin molecules and the possibility 
of a surface active species, in the sandwich compound could be cited as the reason for the 
lack of well-defined redox peaks.  
Finally, there seems to be a dynamic equilibrium which dictates the identity of the 
 63
 64
polyoxometalate species at any given condition of concentration, pH and whether the 
species is in solution or on a surface. 
References: 
1. Pope, M. T., Heteropoly and Isopoly Oxometalates. Springer-Verlag: 1983. 
2. Pope, M. T.; Muller, A.,Angew.chem.Int.Ed.Engl. 1991, 30, 34-48. 
3. Sadakane, M.; Steckhan, E.,Chem. Rev 1998, 98, 219-237. 
4. Keita, B.; Nadjo, L.,Journal of Molecular Catalysis a-Chemical 2007, 262, (1-2), 190- 
    215. 
5. Keita, B.; Mbomekalle, I. M.; Nadjo, L.; Haut, C.,Electrochemistry Communications  
   2004, 6, (10), 978-983. 
6. Ruhlmann, L.; Nadjo, L.; Canny, J.; Contant, R.; Thouvenot, R.,European Journal of  
   Inorganic Chemistry 2002, (4), 975-986. 
7. Keita, B.; Abdeljalil, E.; Nadjo, L.; Contant, R.; Belgiche, R.,Electrochemistry  
   Communications 2001, 3, (2), 56-62. 
8. Keita, B.; Mbomekalle, I. M.; Lu, Y. W.; Nadjo, L.; Berthet, P.; Anderson, T. M.; Hill,  
    C. L.,European Journal of Inorganic Chemistry 2004, (17), 3462-3475. 
9. Keita, B.; Abdeljalil, E.; Nadjo, L.; Contant, R.; Belghiche, R.,Langmuir 2006, 22,  
   (25), 10416-10425. 
10. Keita, B.; Mbomekalle, I. M.; Nadjo, L.; Anderson, T. M.; Hill, C. L.,Inorganic  
    Chemistry 2004, 43, (10), 3257-3263. 
11. Keita, B.; Contant, R.; Mialane, P.; Secheresse, F.; De Oliveira, P.; Nadjo, 
     L.,Electrochemistry Communications 2006, 8, (5), 767-772. 
12. Anderson, T. M.; Fang, X. K.; Mbomekalle, I. M.; Keita, B.; Nadjo, L.; Hardeastle,  
 65
      K. I.; Farsidjani, A.; Hill, C. L.,Journal of Cluster Science 2006, 17, (2), 183-195. 
13. Duncan, D. C.; Hill, C. L.,Journal of the American Chemical Society 1997, 119, (1),  
      243-244. 
14. MeEvoy, A. J.; Gratzel, J.,J.Electroanal.Chem 1986, 209, 391. 
15. B.Keita; L.Nadjo,Chem. Phys 1989, 22, 77. 
16. Keita, B.; Nadjo, L.,Curr. Top. Electrochem 1993, 2, 77. 
17. Keita, B.; Nadjo, L.; Belhouari, A.; Constant, R.,J.Electroanal.Chem 1995, 381, 243. 
18. Toth, J. E.; Anson, F. C.,Journal of Electroanalytical Chemistry 1988, 256, (2), 361- 
      370. 
19. Ritorto, M. D.; Anderson, T. M.; Neiwert, W. A.; Hill, C. L.,Inorganic Chemistry  
    2004, 43, (1), 44-49. 
20. Romo, S.; Jorge, F.; Miquel, M. J.; Keita, B.; Nadjo, L.; Coen, G.; Josep, P.,Inorg.  
     Chem 2007, 46, 4022. 
21. Lopez, X.; Bo, C.; Poblet, J. M.,J. Am. Chem. Soc. 2002, 124, 12574. 
22. Cadot, E.; Fournier, M.; Teze, A.; Herve, G.,Inorg. Chem. 1996, 35, 282. 
23. Rong, C. Y.; Pope, M. T.,J. Am. Chem. Soc. 1992, 114, 2932. 
24. Baker, L. C. W.; Baker, V. E. S.; Eriks, K.; Pope, M. T.; Shibatha, M.; Rollins, O.  
     W.; Fang, J. H.; Kohn.L.L,J. Am. Chem.Soc 1966, 88, 2329. 
25. Zhang, X. Y.; Jameson, G. B.; O?Connor, C. J.; Pope, M. T.; .Polyhedron 1996, 15,  
      917-922. 
26. Keita, B.; Lu, Y. W.; Nadjo, L.; Contant, R.,European Journal of Inorganic 
     Chemistry 2000, (12), 2463-2471. 
27. Bi, L. H.; Kortz, U.; Keita, B.; Nadjo, L.,Dalton Transactions 2004, (20), 3184-3190. 
 66
28. Li-Hua Bi, U. K., Bineta Keita and Louis Nadjo,Dalton Transactions 2004. 
29. Cheng, L.; Dong, S.,Journal of Electroanalytical Chemistry 2000, 481, (2), 168-176. 
30. Liu, J. Y.; Cheng, L.; Li, B. F.; Dong, S. J.,Langmuir 2000, 16, (19), 7471-7476. 
31. Liu, J. Y.; Cheng, L.; Dong, S. J.,Chemical Journal of Chinese Universities-Chinese  
     2001, 22, (10), 1641-1644. 
32. Cheng, L.; Niu, L.; Gong, J.; Dong, S. J.,Chemistry of Materials 1999, 11, (6), 1465- 
     1475. 
33. Bi, L. H.; Kortz, U.; Dickman, M. H.; Keita, B.; Nadjo, L.,Inorganic Chemistry 2005,  
     44, (21), 7485-7493. 
34. Constant, R.,Canadian Journal of Chemistry-Revue Canadienne De Chimie 1987, 65,  
       568. 
35. Demir, U.; Shannon, C.,Langmuir 1996, 12, 6091-6097. 
36. Sanchez-Maestre, M.; Rodriguez-Amaro, R.; Munoz, E.; Ruiz, J.; Camacho,  
     L.,Journal of Electroanalytical Chemistry 1994, 373, 31. 
37. Bosco, E.; Rangarajan, S. K.,Journal of Electroanalytical Chemistry 1981, 129, 25. 
38. Bi, L. H.; Chubarova, E. V.; Nsouli, N. H.; Dickman, M. H.; Kortz, U.; Keita, B.; 
      Nadjo, L.,Inorganic Chemistry 2006, 45, (21), 8575-8583. 
39. Rong, C. Y.; Pope, M. T.,Journal of the American Chemical Society 1992, 114, (8),  
      2932-2938. 
40. Liu, H. Z.; Sun, W. L.; Yue, B.; Jin, S. L.; Deng, J. Q.; Xie, G. Y.,Synthesis and  
     Reactivity in Inorganic and Metal-Organic Chemistry 1997, 27, (4), 551-566. 
41. Sun, W. L.; Yang, F.; Liu, H. Z.; Kong, J. L.; Jin, S. L.; Xie, G. Y.; Deng, J.  
     Q.,Journal of Electroanalytical Chemistry 1998, 451, (1-2), 49-57. 
 67
42. Wenliang Sun, H. L., Jilie Kong, Gaoyang Xie, Jiaqi Deng,Journal of  
     Electroanalytical Chemistry 1997, 437, 67-76. 
43. Keita, B.; Mbomekalle, I. M.; de Oliveira, P.; Ranjbari, A.; Justum, Y.; Nadjo, L.;  
      Pompon, D.,Journal of Cluster Science 2006, 17, (2), 221-233. 
44. Geletii, Y. V.; Bailey, A. J.; Cowan, J. J.; Weinstock, I. A.; Hill, C. L.,Canadian  
     Journal of Chemistry-Revue Canadienne De Chimie 2001, 79, (5-6), 792-794. 
45. Raoof, J. B.; Ojani, R.; Beitollahi, H.,Electroanalysis 2007, 19, (17), 1822-1830. 
46. Keita, B.; Zhang, G. J.; Dolbecq, A.; Mialane, P.; Secheresse, F.; Miserque, F.;  
      Nadjo, L.,Journal of Physical Chemistry C 2007, 111, (23), 8145-8148. 
47. Keita, B.; Essaadi, K.; Nadjo, L.; Desmadril, M.,Chemical Physics Letters 1995, 237,  
     (5-6), 411-418. 
48. Tourne, C. M.; Tourne, G. F.; Zonnevijlle, F.,J.Chem.Soc.Dalton Trans 1991, (1),  
      143-155. 
49. Finke, R. G.; Droege, M.; Hutchinson, J. R.; Gansow, O.,J. Am. Chem.Soc 1981, 103,  
     1587-1589. 
 
 
CHAPTER 4 
DIRUTHENIUM SUBSTITUTED POLYOXOMETALATE AS AN INORGANIC 
ELECTROCATALYST FOR OXYGEN GENERATION 
4.1. Introduction 
Electrocatalytic oxygen generation is a very important reaction in 
electrochemistry and it plays an important role in wide ranging applications from 
industrial metallurgy to regenerative fuel cell applications
1
. Oxygen evolution reaction is 
a very demanding reaction and hence it is highly sensitive to the nature and structure of 
electrocatalysts. The very early oxygen electrodes were based on nickel and nickel 
oxides
2
, and subsequently RuO
2
, IrO
2
, perovskites and mixed oxide systems were 
employed
3
. These oxides have been employed as anodes of water electrolyzers, in metal 
electrowinning processes, cathodic protection, in electro-organic reductions etc.  
The importance of oxygen evolution during the light-assisted decomposition of 
water has been a major driving force in the direction of alternative and clean energy 
systems based on solar energy. In this regard, a lot of attention has been devoted to 
homogeneous water oxidation complexes in the scientific community to mimic the 
oxygen evolving center (OEC) of photosystem II (PS-II), present in green plants. The 
active component for this water oxidation reaction has been identified as a polynuclear 
Ca-Mn
4
 complex with dimeric di-?-oxo Mn units
4
.  
22
+ -
               2H O    O  + 4H + 4e                                        (4.1)?
 
 68
 69
Thought very ubiquitous in nature, this energy intensive reaction [E? =1.23 V (vs 
SHE) at pH =0.0],  has tremendous molecular complexity from a mechanistic 
perspective, since it involves the transfer of four electrons to an oxidant, coupled with the 
release of four protons and ultimately the formation of an O-O bond. In this scenario, 
there are very few well-defined complexes that can oxidize water dioxygen in a 
homogeneous phase and in the absence of light, such as the one occurring in the PS-II 
system .Several di- and tetra nuclear metal complexes have been synthesized and tested 
as models of the OEC complex, and four electron oxidation of water has been achieved 
with only very few complexes, namely about two Mn complexes (not very stable) and a 
number of Ru complexes (more robust). From the review by Llobet et al, of all the Ru 
complexes employed for water oxidation, there are two distinctive types, complexes 
containing Ru-O-Ru bridging group and those without this oxo-bridge
5-7
.   
Till date only a very few totally inorganic catalysts
8, 9
 for oxygen evolution are 
known, and of these, transition metal substituted polyoxometalate clusters are the most 
predominant group of compounds
10
. The chemistry of transition metal substituted 
polyoxometalate is highly developed and much of the interest in recent years is driven by 
their prospects as ?green? oxidation catalysts and as model transition metal oxide 
surfaces
11-14
. 
Within the transition metal substituted polyoxometalates, the sandwich type 
structures, described in the earlier chapters are very important. The sandwich type 
polyoxotungstates consisting of two trivacant Keggin units connected by a central metal 
belt of four transition metals (WZn
3
), first synthesized by Tourne et al
15
 have been 
employed in many catalytic applications via the substitution of different transition metals. 
 70
Recently Neuman et al 
16
 synthesized a di-Ru substituted compound, 
[WZnRu
2
(OH)(H
2
O)(ZnW
9
O
34
)
2
]
11?  
to be employed as organic oxidation catalyst under 
different solvent conditions. 
Ruthenium substituted polyoxometalates have a special importance because of the 
unique redox and catalytic activities of the element, ruthenium
17
. Until now, only a few 
ruthenium based polyoxotungstates have been studied for their catalytic and 
electrocatalytic properties
18-28
. The main reason for the scarcity of well-defined 
ruthenium substituted structures is that the synthesis of these structures is not straight-
forward. This is due to the lack of a pure, well characterized precursor which is water-
soluble, air stable and also labile to release the Ru atoms in solution to be available for 
substitution. Of the compounds available commercially, RuCl
3
.nH
2
O has been employed 
extensively. However, this compound is actually a mixture of mono and polymetallic 
oxides, hydroxides and chlorides of ruthenium in the +3 and +4 oxidation states
29, 30
. 
Table 4.1 shows the collection of ruthenium polyoxometalates and their corresponding 
precursor molecules observed from literature results. 
 
 
 
 
 
 
 
 
 71
Table 4.1 Literature Overview of Ruthenium Polyoxometalate Synthesis with Respect to 
the nature of precursor 
Ru Precursor Polyoxometalate
Precursor 
Ru-substituted Compound Literature 
RuCl
3
?nH
2
O [SiW
11
O
39
]
8?
[Ru
III
(H
2
O)SiW
11
O
39
]
5?
Neuman et 
al
31-33
 
RuCl
3
?nH
2
O [P
2
W
17
O
61
]
10-
[O{Ru
IV
Cl(?
2
- 
P
2
W
17
O
61
)}
2
]
16?
Finke et al
34
 
[Ru
II
(H
2
O)
6
][C
7
H
7
SO
3
]
2
[PW
11
O
39
]
7?
[Ru
III
(H
2
O)PW
11
O
39
]
4?
Pope et al
35
 
[Ru
II
(H
2
O)
6
][C
7
H
7
SO
3
]
2
[P
2
W
17
O
61
]
10-
[?2-Ru
III
(H
2
O)P
2
W
17
O
61
]
7?
Pope et al
35
 
Ru(acac)
3
,
 
hydrothermal 
[SiW
11
O
39
]
8?
[Ru
III
(H
2
O)SiW
11
O
39
]
5?
Higashijima
36
 
Ru(acac)
3
, 
hydrothermal  
[SiW
11
O
39
]
8?
[SiW
11
O
39
- 
Ru
IV
ORu
III
SiW
11
O
39
]
11-
Sadakane
37
 
cis-Ru(dmso)
4
Cl
2
[WZn
3
(H
2
O)
2 
(ZnW
9
O
34
)
2
]
12?
[WZnRu
III
2
(OH)(H
2
O)(ZnW
9
O
34
)
2
]
11?
Neuman et 
al
16
 
Same as above  [?2-RuIII(H2O)P2W17O61]7? Nomiya et 
al
22
 
Same as above 
Microwave assisted  
hydrothermal 
[PW
11
O
39
]
7-
[Ru
II
(dmso)PW
11
O
39
]
5?
Bonchio et 
al
38
 
Same as above  [HW
9
O
33
Ru
II
2
(dmso)
6
]
7?
Li-Hua Bi 
39
 
Same as above  [Ru(dmso)
3
(H
2
O)XW
11
O
39
]
6-
Li-Hua Bi 
28
 
 
General Considerations  
From an electrochemical perspective, the anodic reaction of oxygen evolution has 
a rather high overvoltage on most electrode materials. In comparison to the often studied 
 72
four electron reduction of molecular oxygen (as in fuel cells), there are no strong oxygen-
to-oxygen bond scission involved during water oxidation. From the review by Yeager et 
al
40
, some very interesting aspects about the oxygen evolution reaction can be listed as 
follows.  
i. Oxygen evolution reaction occurs from either OH
-
 or H
2
O in aqueous solutions 
ii. Oxygen evolution reaction in aqueous solutions occur in the potential regime 
where most metal electrodes are not corrosion resistant and hence the need for 
novel electro-catalytic materials 
iii. It is extremely difficult to detect the intermediates of water oxidation such as 
H
2
O
2 
or the corresponding HO
2
-
, since they are very unstable in the potential 
regime of oxygen generation 
iv. In the case of metal based electrocatalysis, an intermediate step of metal oxide 
formation is always observed before oxygen evolution reaction  
4.2. Materials and Methods 
Diruthenium substituted sandwich type polyoxometalate was prepared according 
to methods described in previous chapters. Oxygen generation experiments were 
performed with gold as working electrode, Pt mesh as counter electrode and Ag wire as 
quasi-reference electrode. The supporting electrolyte is phosphate buffer solution at 
pH=8. 
4.3. Results and Discussion 
4.3.1. Elemental analysis 
Elemental analysis determinations were made using energy dispersive X-ray 
(EDX) spectroscopy carried out on the same single crystals that were used for the XRD 
 73
measurements discussed above. For each single crystal sample, three iterations were 
performed and the results for all samples were pooled. Quantitation was carried out for 
Na, Ru, Zn and W. In the table below, averaged experimental data are compared to 
values calculated using the formulation Na
14
[Ru
III
2
Zn
2
(H
2
O)
2
(ZnW
9
O
34
)
2
]. 
Table 4.2 EDX Analysis of single crystal specimen of Ru2POM 
Element Calc weight % Exptl weight % Exptl mol % 
Na 6.17 5.80 25.22 
Ru 3.88 3.79 3.75 
Zn 5.01 4.92 7.52 
W 66.40 64.03 34.84 
 
The experimental W/Zn and Zn/Ru molar ratios are 4.63 and 2.01, respectively. 
These compare well with the above formulation, for which W/Zn = 4.50 and Zn/Ru = 
2.00. The predicted W/Zn molar ratio for Neuman?s formulation
16
 
(Na
11
[ZnWRu
2
(OH)(H
2
O)(ZnW
9
O
34
)
2
) is 6.33, while the Zn/Ru molar ratio is 1.5.  
4.3.2. UV-VIS Spectroscopy for pH Stability 
The UV-Vis electronic spectroscopy of the compounds in neutral conditions, 
shows the general absorption at 300 nm (attributable to all Keggin systems), caused by 
the W-O-W charge transfer bands
16
. In addition, the ruthenium substituted Ru2POM 
exhibits a strong peak at 435nm as shown in Fig.4.1, attributable to the oxygen to 
ruthenium charge transfer band. Effect of change in pH has been studied via the UV-VIS 
spectroscopy, as shown in Fig.4.1, shows that the 435 nm peak does not undergo major 
changes and this shows that from pH 4 to 8, the compound exhibits a stable structure. 
UV-Vis electronic spectroscopy has been extensively used in polyoxometalate 
research to establish the stability of transition metal substituted POMs under different pH 
conditions. Since the W-O-W charge transfer bands are characteristic features of Keggin 
based units, the presence of these strong absorption bands can be useful for determining 
the structural integrity of that particular compound in a given solution. UV-VIS can also 
be employed to determine the degree of completion of a given reaction, by observing the 
charge transfer bands of the POMs.  
 
Fig.4. 1 pH stability analysis using UV-Vis spectroscopy in 0.5M Na
2
SO
4
 (pH adjusted 
by HCl and NaOH) 
 
 
 
 74
4.3.3. Electrochemical Characterization 
Cyclic Voltammetric Measurement as an Evidence for pH Stability 
Au electrodes were employed as working electrodes, together with Pt mesh and a 
Ag/AgCl reference electrode. Ru
2
POM was dissolved in Ar saturated 0.5M Na
2
SO
4
 at a 
concentration of 0.2mM and this solution was employed as the electrolyte under a 
positive pressure of Ar. A non-buffered 0.5 M Na
2
SO
4
 was chosen to determine the effect 
of pH changes. Addition of H
2
SO
4
 and NaOH was performed to achieve the required pH 
condition.  
 
Fig.4. 2 Cyclic Voltammetric studies depicting pH stability in terms of W redox peaks. 
At pH=9, the ?W? reduction peaks are not well-defined, suggesting the breakdown of the 
compound. WE = Au, CE = Pt mesh, RE = Ag/AgCl, Scan rate 10mV/s. 
 75
Cyclic voltammetry has been employed to determine the redox features of 
Ru
2
POM as a function of pH. From Fig.4.2, the two tungsten redox peaks seen at -
0.33mV and -0.48 mV are well-defined in the pH conditions of 4 and 7.4. In the case of 
pH 9, the reduction peaks have lost their shape and this can correspond to some 
decomposition of the compound. As seen by Tourne, the stability window for these 
sandwich compounds
15, 41
 is usually from pH 4 to 8, and our electrochemical data shows 
that in the case of Ru
2
POM, we might be reaching the limits of stability at pH of 9. 
4.3.4. Electrocatalysis for Oxygen Generation Reaction 
 
Fig.4. 3 Oxygen Evolution Reaction as a function of pH in 0.2 mM Ru
2
POM. Au as  
WE, Pt Mesh as CE and Ag/AgCl as RE (represented w.r.t RHE) 
  
 76
 77
 From Fig.4.3 we can identify the effect of pH variation with the onset of the 
oxygen evolution reaction. The last Ru oxidation peak coincides with the oxygen 
generation curve.  
Electrochemical generation of O
2
 was investigated using pulsed voltammetry. In 
the first set of experiments, the amount of O
2
 produced was measured using a Clark 
sensor fabricated with a 500 ?m diameter Pt disk. The sensor assembly was separated 
from the test solution with an oxygen-permeable Teflon membrane and held in close 
physical proximity to a polycrystalline Au anode. The Clark sensor and Au anode served 
as dual working electrodes in a four-electrode electrochemical cell in which a Pt wire was 
the counter electrode and a Ag wire served as a quasi-reference electrode (QRFE). The 
potentials of the Clark sensor and the Au anode were controlled using a conventional 
bipotentiostat circuit. The potential of the Ag QRFE was calibrated against a standard 
hydrogen electrode (SHE), and all potentials are referenced to the H
+
/H
2
 couple.  
The supporting electrolyte was a pH 8 phosphate buffer in which the analytical 
concentration of phosphate was 0.10 M. O
2
 generation experiments were carried out in 
solutions that had been purged with ultrahigh purity N
2
 for at least 30 min, and that were 
maintained under a N
2
 atmosphere during all subsequent measurements. The Au anode 
was stepped from a rest potential of +0.2 V (vs. H
+
/H
2
) to a series of increasingly positive 
potentials. The duration of each voltage pulse was 5 sec; after each pulse, the potential 
was stepped back to +0.2 V to re-establish the baseline response of the O
2
 sensor. The 
amperometric response of the Clark electrode was measured continuously as the Au 
anode was subjected to this potential program, and is plotted as a function of time in 
Fig.4.4. Trace A shows the response of the Au anode (no catalyst present) to a series of 
 78
voltage pulses. No measurable O
2
 generation is observed from naked Au until a potential 
of greater than +1.05 V is applied (not shown), consistent with the known behavior of Au 
anodes. The narrow current spikes that are observed in this data set are due to capacitive 
charging that occurs when the working electrode is subjected to a voltage step, and are 
not faradic. Trace B shows the response of the system to an identical potential program 
after the addition of the un-substituted di-ZnPOM (formal concentration 2 ?M) to the 
electrochemical cell. The behavior of this system is identical to the behavior of the Au 
anode alone, indicating that the di-ZnPOM is not catalytically active for O
2
 generation. 
Trace C shows the response of the Clark sensor after the addition of the di-RuPOM (2 
?M formal concentrations) to pure electrolyte.  
Electrochemically driven generation of O
2
 by this system is clearly observed as 
negative-going current spikes in phase with the applied potential pulses. Negative 
changes in current correspond to O
2
 generation and positive-going currents correspond to 
a decrease in the amount of O
2
 at the sensor. It is clear that O
2
 generation is only 
observed when the working electrode potential is stepped positive from the rest potential; 
when the potential is stepped back to +0.2 V, the response of the Clark sensor returns 
rapidly to its baseline value. Finally, trace D shows the response of the Clark sensor in a 
solution containing a mono-substituted RuPOM catalyst, [PW
11
O
39
Ru
III
(H
2
O)]
4?
, a 
compound first synthesized by Pope and co-workers that exhibits a ?lacunary? Keggin-
type structure
35
. Interestingly, no catalytic activity for O
2
 generation is observed for this 
compound. These findings are consistent with a reaction mechanism involving oxygen-
containing species bound to adjacent Ru sites combining to form molecular oxygen. 
 
Fig.4. 4 Clark Sensor response for the measurement of electrocatalytic generation of 
oxygen from A) Au anode, B) Au anode + 2mM Zn
2
POM, C) Au anode and  Ru
2
POM, 
D) Au anode and mono Ru substituted Keggin POM 
Further characterization of this system was undertaken using a rotating ring disk 
electrode assembly, Fig.4.6. The Pt ring was maintained at a constant potential of  ?0.20 
V while the potential of the Au disk was stepped to increasingly positive voltages (pulse 
duration, 5 s). O
2
 generated at the Au disk was collected at the Pt ring. Mass transport 
from the disk to the ring was controlled by the electrode rotation rate. The disk response 
 79
 80
consists of an initial rapid positive (i.e., anodic) increase in current, followed by a slower, 
quasi-Cottrell (i.e.,i ? t
-1/2
) decrease (inset). The current-time response is similar to that of 
a stationary electrode because the voltage pulses were kept shorter than the time required 
to establish a steady state current in order to prevent the accumulation of excess oxygen. 
When the potential is stepped back to its base value, a negative-going current is 
observed that is due to reduction of O
2
 in the double layer. The response of the Pt ring to 
this voltage program is shown in the lower trace; the pulse shapes are qualitatively 
similar to those seen for the disk. A small but noticeable dip in the ring response occurs at 
the same time point as the cathodic disk current reaches its maximum value, and is due to 
the disk shielding the ring.  
An approximate E
1/2
 for the electrocatalytic generation of O
2
 of +0.750 V was 
calculated from the disk current-potential data, and is consistent with the thermodynamics 
of oxygen generation (E? = +0.760 V vs. SHE at pH 8) and with the redox properties of 
the di-RuPOM electrocatalyst.  
 
 
 
 
 
 
Fig.4. 5 Schematic representation of Rotating Disc Electrode (RRDE) Assembly together 
with the Bipotentiostat arrangement. Working electrode = polycrystalline Au electrode, 
Sensing element ? Clark sensor or the Pt ring of the RRDE. Ag wire Quasi-reference 
electrode (QRFE) was employed. The Counter Electrode (CE) was a high surface area 
platinum mesh. The second figure is the close-up photograph of the RRDE assembly.  
 
 
 
 
 81
 
Fig.4. 6 Electrochemical generation of O
2
 at pH 8 using Ru
2
POM catalyst, which is 
characterized using a Rotating Ring Disc Electrode (RRDE). Disc current (upper trace) 
and ring current (lower trace) were acquired simultaneously. The rotation rate was 
1600RPM and the geometrical area of the disc was 0.27 cm
2
. The inset shows an 
expanded view of one set of pulses. 
 4.5. Conclusion 
In conclusion, we have demonstrated that the di-Ru-substituted POM 
{[Zn
2
Ru
2
(OH)(H
2
O)](ZnW
9
O
34
)
2
}
11-
 functions as a catalyst for the electrochemical 
 82
 83
generation of O
2
. The structural stability of Sandwich type structures have been tested 
both by electronic spectroscopy and electrochemistry. Our results indicate that adjacent 
Ru sites are necessary to observe O
2
, which points to a reaction pathway involving two 
Ru-bound oxygen species combining to form O
2
. The observed kinetics are similar to 
those found previously for perovskite materials, suggesting that adsorbed monolayers of 
this POM would serve as robust models of bulk oxide electrodes.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 84
References  
1. Chen G; Delafuente D; Sarangapani S; Mallouk T,Catal. Today 2001, 67, 341. 
2. Justi, E. W. W., E. A., Kalte Verbrennung-fuel Cells. Franz Steiner Verlag,  
   Wiesbaden, : 1962; p 248. 
3. Damjanovic, A., in: Bockris, J.O?M.; Conway, B.E. (Eds), Modern Aspects of  
   Electrochemistry, . Plenum Press, New York, : 1969; Vol. 5, p 369. 
4. Yachandra V K; De Rose V J; Latimer M J; Mukerji I; Sauer K; Klein M P,Science  
    1993, 260, 675. 
5. Romero, I.; Rodriguez, M.; Sens, C.; Mola, J.; Kollipara, M. R.; Francas, L.; Mas- 
   Marza, E.; Escriche, L.; Llobet, A.,Inorganic Chemistry 2008, 47, (6), 1824-1834. 
6. Rodriguez, M.; Romero, I.; Sens, C.; Llobet, A.,Journal of Molecular Catalysis a 
  -Chemical 2006, 251, (1-2), 215-220. 
7. Sens, C.; Romero, I.; Rodriguez, M.; Llobet, A.; Parella, T.; Benet-Buchholz,  
   J.,Journal of the American Chemical Society 2004, 126, (25), 7798-7799. 
8. Yagi, M.; Yamaguchi, T.; Kaneko, M.,Journal of Molecular Catalysis a-Chemical  
   1999, 149, (1-2), 289-295. 
9. Ogino, O.; Nagoshi, K.; Yagi, M.; Kaneko, M.,Journal of the Chemical Society- 
    Faraday Transactions 1996, 92, (18), 3431-3434. 
10. Sartorel, A.; Carraro, M.; Scorrano, G.; De Zorzi, R.; Geremia, S.; McDaniel, N. D.;  
     Bernhard, S.; Bonchio, M.,Journal of the American Chemical Society 2008, 130, (15),  
     5006-+. 
11. Baker, L. C. W.; Glick, D. C., Chem. Rev. 1998, 3, 98  
12. Hill, C. L.; Prosser-McCartha, C. M.,Coord. Chem. Rev 1995, , 143, 407. 
 85
13. Sadakene, M.; Steckhan, E.,Chem. Rev. 1998, 98, 219. 
14. Kim, J.; Gewirth, A. A.,Langmuir 2003, 19, 8934. 
15. Tourne, C. M.; Tourne, G. F.; Zonnevijlle, F.,J.Chem.Soc.Dalton Trans 1991, (1),  
      143-155. 
16. Neuman, R.; Khenkin, A. M.,Inorganic Chemistry 1995, 34, 5753-5760. 
17. Naota T; Takaya H; MurahashiS,Chem. Rev. 1998, 98, 2599. 
18. Bi, L. H.; Kortz, U.; Keita, B.; Nadjo, L.,Dalton Transactions 2004, (20), 3184-3190. 
19. Khenkin, A. M.; Shimon, L. J. W.; Neumann, R.,Inorganic Chemistry 2003, 42, (10), 
      3331-3339. 
20. Qi, J. Y.; Qiu, L. Q.; Lam, K. H.; Yip, C. W.; Zhou, Z. Y.; Chan, A. S. C.,Chemical  
     Communications 2003, (9), 1058-1059. 
21. Cheng, L.; Cox, J. A.,Chemistry of Materials 2002, 14, (1), 6-+. 
22. Nomiya, K.; Torii, H.; Nomura, K.; Sato, Y.,Journal of the Chemical Society-Dalton 
     Transactions 2001, (9), 1506-1512. 
23. Bagno, A.; Bonchio, M.; Sartorel, A.; Scorrano, G.,European Journal of Inorganic 
     Chemistry 2000, (1), 17-20. 
24. Neumann, R.; Dahan, M.,Journal of the American Chemical Society 1998, 120, (46),  
      11969-11976. 
25. Neumann, R.; Dahan, M.,Polyhedron 1998, 17, (20), 3557-3564. 
26. Neumann, R.; Dahan, M.,Nature 1997, 388, (6640), 353-355. 
27. Neumann, R.; Khenkin, A. M.; Dahan, M.,Angewandte Chemie-International Edition 
     in English 1995, 34, (15), 1587-1589. 
28. Li-Hua Bi, U. K., Bineta Keita and Louis Nadjo,Dalton Transactions 2004. 
 86
29.Seddon, E. A.; Seddon, K. R., The Chemistry of Ruthenium. Elsevier, New York:  
    1984; p 156. 
30. Rard, J. A.,Chem. Rev. 1985, 85, 1. 
31. Neumann, R.; Abu-Gnim, C. J.,J. Chem. Soc., Chem. Commun 1989, 1324. 
32. Neumann, R.; C. J. Abu-Gnim,J. Am. Chem. Soc. 1990, 112, 6025. 
33. Filipek, K., Inorg. Chim. Acta 1995, 231, 237. 
34. W. J. Randall; T. J. R. Weakley; R. G. Finke,Inorg. Chem. 1993, 32, 1068. 
35. Rong, C. Y.; Pope, M. T.,J. Am. Chem. Soc. 1992, 114, 2932. 
36. Higashijima, M.,Chem. Lett. 1999, 1093. 
37. Sadakane, M.; M. Higashijima,J. Chem. Soc., Dalton Trans 2003, 659. 
38.  A. Bagno; M. Bonchio; Sartorel, A.; Scorrano, G., Eur. J. Inorg. Chem 2000, 17. 
39. L. Bi; F. Hussain; U. Kortz; Sadakane, M.; Dickman, M. H.,Chem. Commun 2004,  
     1420. 
40.Tarasevich, M. R.; sadkowski, A.; Yeager, E., 1983; Vol. 7, p 301. 
41. Jabbour, D.; Keita, B.; Israel-Martyr; Mbomekalle; Nadjo, L.; Kortz, U.,European 
     Journal of Inorganic Chemistry 2004, 2004, (10), 2036-2044. 
 87
CHAPTER 5 
ELECTROCHEMICAL SURFACE PLASMON RESONANCE STUDIES TO 
EVALUATE POLYOXOMETALATE ADSORPTION ON GOLD ELECTRODES 
5.1. Introduction 
In the previous chapters, we have seen the unique properties and catalytic 
applications of sandwich type polyoxometalates. The application of polyoxometalates as 
homogenous redox mediators has been very well established and in order to achieve 
industrial relevance in fuel cells, batteries or other electrosynthetic uses, there is a need to 
transfer the catalytic properties of polyoxometalates onto solid surfaces. In this regard, 
entrapment of POMs in polymer matrices and direct electrodeposition onto electrodes has 
been successfully performed
1, 2
. Several studies of Keggin adsorption onto gold 
electrodes during electrochemical cycling have been reported 
3-5
. In all these studies, the 
adsorption of POM species has been monitored using electrochemical quartz crystal 
microbalance (EQCM) method. Using this method, many important insights have been 
made about the behavior of polyoxometalates in the presence of an electric field. 
In the present work, we are interested in the application of electrochemical 
surface plasmon resonance as an elegant complementary technique to study 
polyoxometalate behavior. In recent years, surface plasmon resonance (SPR) has been 
identified as a potential technique for the analysis and characterization of ultra-thin films, 
interfaces and kinetic processes at surfaces
6
 . When combined with electrochemistry, the 
 88
electrochemical surface plasmon resonance (ESPR) technique has been demonstrated as a 
powerful technique for the simultaneous characterization and manipulation of 
electrode/electrolyte interfaces
7-9
. Electrochemical and optical properties of nanometer-
scale films can be studied simultaneously using the gold sensing substrate as a working 
electrode.  
5.2. Theoretical Background on Surface plasmon resonance 
Surface plasmon resonance (SPR) has been used as a sensitive technique for 
detecting analytes adsorbed onto a metal surface
10
. It has been widely employed as an 
analytical tool in biomedical and pharmaceutical applications to screen and identify 
biological and chemical reagents. SPR has been defined
11
 as an optical phenomenon, 
which occurs when TM-polarized light undergoes total internal reflection at certain 
angles of incidences (resonance angle) at an interface between two media of differing 
refractive indices and resulting in the production of an evanescent wave. Surface 
plasmons are a result of collective oscillations of free electrons in the metal film. When 
these plasmons undergo resonance with the incident light beam, they absorb part of the 
energy of the beam. This phenomenon of resonance produces the evanescent wave, which 
propagates perpendicular to the surface of the metal film. Usually in a SPR set-up, the 
interface is modified with a thin layer of metal (either Au or Ag ~ 50 nm) and a 
monochromatic p-polarized light is used as a photonic source. In this arrangement, for a 
given thickness of metal in the interface, a corresponding minimum in reflectivity is 
observed at the resonance angle. The resonance angle (or position of the minimum) is 
very sensitive to even minute changes in the refractive index (R.I) of the adjacent 
medium, which is ultimately a function of the change in surface concentration of 
interacting species at the interface. During a typical SPR experiment, the change in 
refractive index is continuously monitored as a function of time. There is a relationship 
between the change in refractive index and the corresponding change in surface analyte 
concentration. For proteins, a value of 1000 RIU (refractive index units) or 0.1? change in 
angle of total internal reflection, is equated to 1ng/mm
2
 change in surface concentration
12
. 
Thus, Surface plasmon resonance (SPR) is one of the most sensitive methods for 
detection of analyte binding events ? protein adsorption on functionalized surfaces, metal 
deposition from solution, adsorption of thiol molecules etc. Using SPR, the surface 
coverage concentration of adsorbed layer can be calculated using the following equation 
5.1. 
                           (5.1) 
( )
()
*
2
eff b
d
a
ab
nn
l
d
nn
? ?
?
??
? ?=
??
?
??? ?
? ?
Where   d
a
 - Thickness of adsorbed layer 
              l
d
 - Characteristic decay length (~307nm) 
              n
eff
 - Effective RI of adlayer (from SPR) 
               n
b 
- Refractive index of electrolyte  
    n
a
 - Refractive index of Polyoxometalates (~2)          
 
 Electrochemical surface plasmon resonance (ESPR) is a relatively novel approach 
to evaluate the behavior of an interface in the presence of an electrical field. The applied 
electric field modulates the observed SPR response as follows:  
 89
?
?
?
?
?
?
?
?
+
?
?
?
?
?
?
?
?
+
?
?
?
?
?
?
?
?
=
?
?
n
c
n
d
c
V
n
c
V
??
321                   (5.2) 
where ? is the reflected light intensity where SPR occurs, n corresponds to the dielectric 
constant of the adsorbed film, d is the thickness of the film, ? is the surface charge 
density of the gold surface and c1, c2 & c3 are constants. Here, ? and ? are wavelength 
dependent while last two terms are wavelength independent. Because of its sensitivity to 
the applied electric field, electrochemical-SPR was largely exploited for investigating 
interfacial reactions. Several studies have been performed to target the detection of heavy 
metal ions
10, 13-15
 and more particularly a combined technique of anodic stripping 
voltammetry (ASV) and surface plasmon resonance
10
 has been developed. Wang et al
16
 
have employed catalyzed metal deposition onto the gold sensing surface of the SPR set-
up to produce selective surfaces for heavy metal ions. The insensitivity of ESPR to 
background currents due to presence of dissolved oxygen and other gases has been 
claimed as an advantage over the traditional anodic stripping voltammetry. An elegant 
approach to modify self-assembled monolayers using electrochemical potential has been 
studied using ESPR
11
. Further considerable insights into electropolymerization of 
conductive polymers such as polyaniline
17-19
 and polypyrrole have been obtained using 
electrochemical SPR. 
 
5.3. Electrochemical Adsorption of Polyoxometalates on electrodes  
Due to the high solubility of polyoxometalates in water, there is a need to 
immobilize the catalytically active species on a solid substrate ? carbon, metal 
nanoparticles or inert alumina or silica. For electro-active species contributing to 
 90
 91
catalysis, there is a possibility of employing electrodeposition to transfer catalytic activity 
from solution to solid substrates. The early evidence for electrodeposition of 
polyoxometalates onto metal and glassy carbon surfaces was provided by Profs. Keita 
and Nadjo
20-24
. Since then several other studies have been performed to elucidate the 
mechanism of electrodeposition of POMs. It has been shown that in the case of Keggin 
molecules, when the potential is scanned until the value corresponding to the eight and 
twelve electron reduction peaks, there is a formation of an adherent film
23
. These films 
have been shown to have good electrocatalytic properties towards hydrogen evolution 
reaction. Despite forming well-ordered two dimensional arrays (via AFM and STM 
studies
25
), the exact chemical nature of these films have not yet been identified. The 
electrode modification has been claimed to have been formed by electrodecomposed 
Keggin structures
3
. Electrochemical quartz crystal microbalance method has been 
employed to demonstrate that oxidized form of the polyoxometalates (SiW
12
O
40
4-
, 
P
2
W
18
O
62
6-
 and H
2
W
12
O
40
6-
) undergo adsorption on gold electrodes. Surprisingly, there is 
an absence of adsorption of species formed during the first two or three tungsten redox 
peaks of these compounds. Protonation of the reduced product has been cited as the 
reason for this observation. An increase in potential beyond the initial redox waves and 
reaching the realm of 6e
-
 reduction has been shown to form thick tungstate films on the 
metal electrode. Some of the basic understandings from their studies are as follows: 
1. To obtain a persistent and adherent film, there is a necessity to fix more than six 
electrons per molecule in the case of [P
2
Mo
18
O
62
]
6- 
, i.e. the electrodeposition 
occurs only when the potential is cycled towards more negative potentials 
corresponding to almost six electron reduction
4
 
 92
2. It has been demonstrated that electrodeposition of catalyst at -1.2 V in acidic 
solutions exhibits good electrocatalytic property for hydrogen evolution reaction, 
but then they do not necessarily maintain the same Keggin or Dawson structure of 
the parent compounds 
3. At very mild pH and potential conditions, composite films of new electroactive 
oxides have been deposited, when the starting materials are molybdenum 
containing tungsten frameworks or fully molybdic species 
4. During electrodeposition, there is a possibility of forming new electroactive 
catalytic species by decomposition and/or polymerization reactions of 
heteropolyanions 
5. Molybdenum based species exhibit more propensity to undergo electrodeposition 
than tungsten based complexes 
6. It has been demonstrated that (W
VI
 , W
V
) or (Mo
VI
, Mo
V
) mixtures can be 
considered as good catalysts and electrocatalysts 
There has been a dearth of studies of electrochemical adsorption of sandwich type 
polyoxometalates in the literature and hence this is an attempt made to illustrate the same 
with electrochemical surface plasmon resonance spectroscopy. 
 
 
5.4. Materials and Methods  
5.4.1 Polyoxometalates and Reagents 
The precursor sandwich type Zn
2
POM and Ru
2
POM structures were prepared as 
described in previous chapters and SiW
12
O
40
4-
, the Keggin compound was purchased 
 93
from Alfa Aesar. Two different electrolytes were chosen, 0.5M Na
2
SO
4 
and 0.1M 
NaClO
4
. Keggin based experiments were performed in pH=4 medium (0.1M NaClO
4
 + 
HClO
4
). 
5.4.2. Electrochemical Surface Plasmon Resonance Set-up 
Spreeta
TM
 units developed by Texas Instrument (TI) were employed as sensors for 
our experiments. Advantages such as robust design, ease of handling and relatively lower 
cost (US25$) have made us choose these Spreeta
TM 
units for our application. The 
sensitivity of these units is in the range of ~3 x 10
-6
 R.I, and they are very well suited to 
address extremely low surface concentrations (Specifically monolayer concentrations in 
our case). For the purpose of adding electrochemical capability to these systems, the 
following modification to the sensor has been performed. A Ni wire (Alfa Aesar, 0.001 
mm thickness) has been attached to the gold sensing surface using silver epoxy (Alfa 
Aesar) to provide a conductive contact. This current collector was glued on to one corner 
of the rectangular gold sensing area, so that there is no hindrance to the regular signal 
generation during sensor operation. A layer of silicone sealant was placed on top of this 
Ag epoxy contact to ensure mechanical stability during experimentation. The sealant was 
allowed to cure overnight and then the system was subjected to pre-treatment of the gold 
surface. A complete experimental set-up is shown in Fig.5.1. 
 
Fig.5. 1 Schematic representation of the experimental set-up showing the modified 
SpreetaTM as the working electrode, Pt mesh as counter electrode and Ag/AgCl as 
Reference electrode. 
5.4.3. Pre-treatment of gold electrode  
The gold sensing surface of the Spreeta
TM
 set-up, which was used as the working 
electrode was rinsed with piranha solution (3:1 ratio of conc. H
2
SO
4
 and 30 wt% H
2
O
2
), 
with utmost care as to not damage the polymer outer walls. After thorough rinsing with 
de-ionized water, the electrode was electrochemically tested for consistent gold oxidation 
and reduction peaks in 0.1M HClO
4
 solution, by cycling from 0.0 V to 1.6 V.  
5.4.4. Electro-adsorption by Potential Cycling of Polyoxometalate Solutions  
The phosphotungstate species containing Keggin structure was initially subjected 
to electrochemical cycling in the modified ESPR set-up. The electrodeposition of  Keggin 
structures have been previously studied by Nadjo et al
20-24
 using electrochemical quartz 
 94
 95
crystal microbalance method (EQCM), and hence well-behaved SiW
12
O
40
4-
 species was 
considered a good starting point to elucidate the characteristics of ESPR. A freshly 
prepared 0.2mM solution of SiW
12
O
40
4- 
in pH 4 medium (0.1M NaClO
4
 + HClO
4
) was 
chosen as the deposition solution. The solution was subjected to 30 minutes of de-
oxygenation using Ar gas bubbling and the experiment was performed under a constant 
blanket of Ar gas. Pt mesh was employed as counter electrode and Ag/AgCl (3M NaCl) 
as reference electrode. All potentials are reported as values with respect to a Reversible 
Hydrogen Electrode (RHE). A potential range of +0.2 V to -0.5 V ( vs RHE) was chosen 
for electrochemical cycling. 
Sandwich type structures were then tested using the same set-up under controlled 
pH and concentration. Freshly prepared solutions of Zn
2
POM and Ru
2
POM were 
subjected to electrochemical cycling between +0.4 V to -0.6 V, to determine the nature of 
electro-adsorption. 
5.4.5. Constant Potential Electrodeposition 
Two distinct potentials of -0.2V and -0.7V (vs RHE) were chosen for constant 
potential electrodeposition. The effect of concentration variation from 0.1 ?M to 10 ?M 
was also tested under these deposition conditions. The results of the constant potential 
electrodeposition studies were used as calibration for the sensing surface. 
5.5. Results and Discussion 
5.5.1 Adsorption of sandwich type polyoxometalates on gold electrodes at open 
circuit voltage 
It has been shown that the most effective means of depositing POMs onto 
electrode surfaces is by simply soaking the electrode in acidified aqueous POM 
 96
solution
26
. Almost all polyoxometalate species undergo spontaneous adsorption at open 
circuit potential onto glassy carbon, gold and even platinum surfaces. This particular 
characteristic has been employed in immobilizing POM species onto traditional porous 
catalytic substrates such as silica, activated carbon and carbon nanotubes. Despite the 
promising nature of these POM films in heterogeneous catalysis, especially in the case of 
electrocatalysis, the monolayer nature of these POMs onto electrode surfaces are not 
robust enough to provide reliable functional surfaces. Hence there is a need to immobilize 
these POM species, by electro-adsorption, electrostatic layer by layer deposition or by co-
deposition with polymer films. In order to understand the true behavior of the sandwich 
polyoxometalates in the presence of an electric field, it is important to differentiate 
between spontaneous adsorption (by physisorption and chemisorption) and 
electrodeposition. Initially, a flow-through Spreeta? system was utilized to investigate 
spontaneous adsorption of POM on gold electrode while simultaneous electrochemical-
SPR batch setup (Fig. 5.1) was exploited for electrical potential influence on POM 
deposition. Control experiments were carried out to ensure no adsorption occurs from 
electrolytes (0.5M Na
2
SO
4
).  
Fig 5.2a shows the spontaneous adsorption of Ru
2
POM to SPR gold surface.  
Following the introduction of Ru
2
POM, an immediate increase in SPR angle was 
observed which is due to bulk refractive index change followed by gradual adsorption 
kinetics. Rinsing the surface with Na
2
SO
4
 removed the bulk refractive index of the 
solution observed during adsorption process and resulted in 24.3 mdeg change. This 
could be attributed to the residual film formed on the surface due to spontaneous 
adsorption of Ru
2
POM. A similar but enhanced mass adsorption (nearly twice) was 
observed for Zn
2
POM species as outlined in fig 5.2b.  
70.4
70.5
70.6
70.7
70.8
70.9
71
71.1
71.2
71.3
71.4
71.5
71.6
10300 10500 10700 10900 11100 11300 11500 11700 11900
Time / sec
R
e
son
a
nce A
n
gl
e,
 
?
sp
r
Na
2
SO
4
Ru
2
POM
a
Na
2
SO
4
 
0
10
20
30
40
50
60
SP
R
 angl
e
 
(
m
d
e
g)
Ru
2
POM Zn
2
POM
b
 
Fig.5.2a. Spontaneous adsorption from a solution of 0.2 mM Ru
2
POM in 0.5 M Na
2
SO
4
. 
b. Comparison of SPR response for Ru
2
POM and Zn
2
POM on gold electrode 
 
 97
 98
5.5.2. Electrodeposition of Sandwich Polyoxometalates  
Calibration of Surface Plasmon Resonance in pure 0.1M HClO
4
 solution 
The ability of the electrochemical-SPR setup to simultaneously monitor the 
electrode interfacial processes was demonstrated by cyclic voltammetry of 0.1M HClO
4
 
in the following Fig.5.3. From the studies made by Iwasaki et al
27
, it has been shown that 
the scan rate of less than 10mV/s must be chosen for simultaneous measurement of CV 
and SPR signals.  
At low potential regimes (0.0 to 0.95V) corresponding to double-layer charging of 
the electrode, SPR response increases with the applied potential and this arises due to 
changes in the surface charge density which is usually indistinguishable in cyclic 
voltammetry. After electrode double layer charging, gold oxidation starts at 1.05V vs 
RHE, in our case we see a composite wave in the CV, which has been shown to be 
comprised of three oxidation peaks (a. deposition of OH in pre-adsorbed anions, b. the 
place exchange reaction of anions with OH, c. the formation of gold oxide
27
). There is a 
single peak in the cathodic part of the scan corresponding to the two electron reduction of 
gold oxide. These observations validate the electrochemical-SPR setup for simultaneous 
monitoring of the interfacial processes.  
0.0 0.5 1.0 1.5 2.0
-150
-100
-50
0
50
100
150
0.0
0.1
0.2
0.3
0.4
Curr
ent
 (
m
A)
Potential / V vs RHE
 
 
??
SP
R
 
 
Fig.5. 3 ESPR response in 0.1 M HClO
4
 superimposed with the corresponding cyclic 
voltammetry. 
 
ESPR Response of Keggin type Polyoxometalates in acidic solution 
A Keggin solution (0.2 Mm SiW
12
O
40
4-
) in a supporting electrolyte of pH 4 was 
chosen to evaluate the response of the electrochemical surface plasmon resonance 
(ESPR) to potential cycling. This Keggin based system has already been studied by Keita 
et al
24
 using electrochemical quartz crystal microbalance (EQCM) method and hence 
there is a possibility of comparing the ESPR response to the literature studies. This 
exercise has been designed to permit the understanding of ESPR response as an indirect 
method to elucidate mass variations. A potential region from +0.4V to -0.3V has been 
chosen for our studies as shown in Fig.5.4. 
 99
-0.4 -0.2 0.0 0.2 0.4
-20
-15
-10
-5
0
5
10
 C
u
rrent ( 
?
A)
Potential / V vs RHE
 I  cycle
 II cycle
 
-0.4 -0.2 0.0 0.2 0.4
0.00
0.05
0.10
0.15
 
??
SPR
Potential / V vs RHE
I  Cycle
II Cycle
 
Fig.5. 4 Electrochemical surface plasmon resonance of 0.2mM Keggin (SiW
12
O
40
4-
) in 
pH4 supporting electrolyte (0.1M NaClO
4
 + HClO
4
) solution. The upper graph shows 
cyclic voltammetry behavior and the bottom graph is the corresponding ESPR response. 
 100
 
 101
Initially, the gold electrode is subjected to a pre-treatment in the supporting 
electrolyte. A potential window from -0.3 to +0.4V was chosen and this corresponds to 
the region of the first redox couple of SiW
12
O
40
4-
. There is no significant change in SPR 
signal (??
SPR
) when the electrode is subjected to potential cycling in the pure supporting 
electrolyte.  In the case of SiW
12
O
40
4-
, the electrochemical behavior is well-known and at 
a pH ~4, the first two redox peaks in the negative direction are mono-electronic, 
corresponding to W (VI) reduction. In Fig.5.4, ??
SPR
 corresponding to this potential cycle 
reveals a steady increase in signal during the forward scan. This could be correlated to 
mass increase corresponding to adsorption of oxidized POM. On the reverse scan, the 
??
SPR 
shows two domains, a. from +0.4 to +0.05V, the variation is very small; b. from 
+0.05 to -0.03V, the region where the POM is reduced, a steady decrease is seen in 
??
SPR
. This decrease could be due to loss of mass from the surface, corresponding to 
desorption of reduced species. From the previous study performed on EQCM, a similar 
response has been reported for the Keggin species, SiW
12
O
40
4-
 and hence we can 
conclude that the SPR response can be correlated to the mass increase in the case of Si 
Keggin. Keita et al
2, 24
 had concluded that the observed mass increase during oxidative 
scan is due to adsorption of oxidized species SiW
12
O
40
4-
 and the mass decrease is due to 
desorption of reduced species SiW
12
O
40
5-
.  
ESPR Response of Sandwich POMs in 0.5M Na
2
SO
4
ESPR studies on the electrochemical behavior of sandwich type polyoxometalates 
have been performed in 0.5 M Na
2
SO
4
 solution as supporting electrolyte. Initially, the 
potential window was chosen in order to address the first two redox features of Zn
2
POM, 
as seen in Fig. 5.5. 
-0.6 -0.4 -0.2 0.0 0.2
-25
-20
-15
-10
-5
0
5
C
urrent 
(
?
A
)
Potential / V vs RHE
 
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
 
??
SPR
Potential / V vs RHE
 
Fig.5. 5 ESPR response corresponding to potential cycling in 0.4mM Zn
2
POM. Upper 
graph shows the cyclic voltammogram and the bottom graph shows the corresponding 
SPR response curve. 
 102
 103
Fig. 5.5 shows the ESPR signal (??
SPR
) response recorded simultaneously with 
the voltammogram shown above for 0.4 mM Zn
2
POM. The signal increases during the 
reduction curve and it slows down, but continues during the subsequent positive going 
scan. On increasing the number of potential scans, SPR signal increased continuously and 
even when the cycling was stopped, it does not fall back to the initial value. This suggests 
that there may be an irreversible electro-adsorbed film, which may be stable under open 
circuit conditions. Since the film is still present in the modifying solution, it will be 
interesting to observe the film stability in pure supporting electrolyte. One interesting 
observation in the cyclic voltammetry of Zn
2
POM in Au working electrode of ESPR set-
up is the decrease of current response with each increasing potential cycle. This behavior 
suggests the formation of films, which maybe insulating in nature. Previous studies
5, 25
 on 
electrodeposition of  Keggin structures and Wells-Dawson have shown films exhibiting 
increase in current.  
The Ru
2
POM compound also exhibited very same growth characteristics, in terms 
of the potential response, but SPR signal increase corresponding to the mass increase was 
about half of that seen in Zn
2
POM growth.  
In order to evaluate the stability of the electro-adsorbed film as a function of 
potential, a 0.4 mM Ru2POM solution was subjected to film growth cycling (from +0.2 
V to -0.7 V vs RHE) for about 10 minutes (curve not shown), and then subsequently, the 
potential was scanned from 0 V to +1.2 V and then back to the negative region of -0.7 V.  
 
-1.0 -0.5 0.0 0.5 1.0 1.5
-30
-20
-10
0
10
20
30
 Curr
ent
 (
?
A)
Potential / V vs RHE
 
-1.0 -0.5 0.0 0.5 1.0 1.5
-0.8
-0.6
-0.4
-0.2
0.0
 
??
SP
R
Potential / V vs RHE
 
Fig. 5.6. Potential Stability testing of the electro-adsorbed film in 0.4 mM Ru
2
POM 
solution in 0.5 M Na
2
SO
4
. 
 
 
 104
 105
Fig.5.6 shows the SPR response corresponding to the potential scan to extremely 
positive values of 1.2 V. The potential scan began at 0.0 V, since there is no film growth 
in this region. On scanning more positive towards the gold oxidation peaks, we see a 
dramatic fall in the SPR signal, which corresponds to desorption of the electro-adsorbed 
film. This observation proves that the film formed by cyclic voltammetry and under short 
time duration (10 minutes), has limitations in terms of potential stability in the gold 
oxidation region. This behavior has been anticipated and previous workers
24, 25
 have tried 
to stay away from this region due to the uncertainty of the gold oxide influence in 
polyanion adsorption. We ventured into this region of high anodic potential to address the 
possibility of employing these electro-adsorbed films as oxygen generation catalysts at 
gold electrodes (as a follow-up to studies in chapter 4). The competition due to sulfate 
anion adsorption in this solution and the effect of different supporting electrolytes could 
be an interesting direction for future studies. 
Conclusions  
Electrochemical surface plasmon resonance spectroscopy has been evaluated for 
polyoxometalate characterization during electrochemical cycling. Interesting insights into 
the stability of the electro-adsorbed films were obtained. Further, a differentiation 
between spontaneous adsorption and electro-adsorption has been proved using ESPR. 
References 
1. Keita, B.; Nadjo, L.,Curr. Top. Electrochem 1993, 2, 77. 
2. Keita, B.; Nadjo, L.; Belhouari, A.; Constant, R.,J.Electroanal.Chem 1995, 381, 243. 
3. Keita, B.; Nadjo, L.; Belanger, D.; Wilde, C. P.; Hilaire, M.,Journal of 
    Electroanalytical Chemistry 1995, 384, 155-169. 
 106
4. Keita, B.; Abdeljalil, E.; Girard, F.; Gerschwiler, S.; Nadjo, L.; Contant, R.; Haut, 
    C.,Journal of Solid State Electrochemistry 1999, 3, 446-456. 
5. Keita, B.; Contant, R.; Abdeljalil, E.; Girard, F. o.; Nadjo, L.,Electrochemistry 
    Communications 2000, 2, 295-300. 
6. W.Knoll,Annual Reviews on physical chemistry 1998, 49, 569. 
7. A. Tadjeddine; D.M. Kolb; Kotz, R.,Surf. Sci. 1980, 101, 277. 
8. J.G. Gordon II; Ernst, S., Surf. Sci. 1980, 101, 499. 
9. Y. Iwasaki; T. Horiuchi; M. Morita; Niwa, O.,Electroanalysis 1997, 9, 1239. 
10. Wang, S.; Forzani, E. S.; Tao, N.,Anal. Chem. 2007, 79, (12), 4427-4432. 
11. S.Balasubramanian; I.B.Sorokulova; V.J.Vodyanoy; A.Simonian, Biosensors and  
      Bioelectronics 2007, 22, 948. 
12. R.J.Green; Frazier, R. A.; K.M.Shakesheff; M.C.Davies; C.J.Roberts; 
      S.J.B.Tendler,Biomaterials 2000, 21, 1823. 
13. Moon, J.; Kang, T.; Oh, S.; Hong, S.; Yi, J.,Journal of Colloid and Interface Science  
     2006, 298, (2), 543-549. 
14. Kang, T.; Moon, J.; Oh, S.; Hong, S.; Chah, S.; Yi, J.,Chemical Communications 
     2005, (18), 2360-2362. 
15. Gu, H.; Ng, Z.; Deivaraj, T. C.; Su, X.; Loh, K. P.,Langmuir 2006, 22, (8), 3929- 
      3935. 
16. Wang, J.; Wang, F.; Zou, X.; Xu, Z.; Dong, S.,Electrochemistry Communications  
     2007, 9, (2), 343-347. 
17. Damos, F. S.; de C?ssia Silva Luz, R.; Tanaka, A. A.; Kubota, L. T.,Journal of 
      Electroanalytical Chemistry 2006, 589, (1), 70-81. 
 107
18. Baba, A.; Tian, S.; Stefani, F.; Xia, C.; Wang, Z.; Advincula, R. C.; Johannsmann, 
D.; Knoll, W.,Journal of Electroanalytical Chemistry 2004, 562, (1), 95-103. 
19. Kang, X.; Jin, Y.; Cheng, G.; Dong, S.,Langmuir 2002, 18, (26), 10305-10310. 
20. Keita, B.; Nadjo, L.,Journal of Electroanalytical Chemistry 1988, 243, (1), 87-103. 
21. Keita, B.; Nadjo, L.,Journal of Electroanalytical Chemistry 1988, 247, (1-2), 157- 
      172. 
22. Keita, B.; Nadjo, L.,Journal of Electroanalytical Chemistry 1990, 287, (1), 149-157. 
23. Keita, B.; Nadjo, L.; Sav?ant, J. M.,Journal of Electroanalytical Chemistry 1988, 
     243, (1), 105-116. 
24. Keita, B.; Nadjo, L.; Contant, R.,Journal of Electroanalytical Chemistry 1998, 443,  
     (2), 168-174. 
25. Abdeljalil, E.; Keita, B.; Nadjo, L.; Contant, R.,Journal of Solid State 
      Electrochemistry 2001, 5, (2), 94-106. 
26. Sadakane, M.; Steckhan, E.,Chem. Rev 1998, 98, 219-237. 
27. Iwasaki, Y.; Tsutomotu, H.; Morita, M.; Niwa, O.,Electroanalysis 1997, 9, (16), 
     1239-1241. 
 
 
 108
CHAPTER 6 
POLYOXOMETALATES AS CO-CATALYSTS FOR OXYGEN REDUCTION 
6.1. Introduction 
In the field of electrochemistry, oxygen reduction is one of the most important 
reactions as it plays an important role in the operation of a fuel cell. Fuel cells, most 
importantly proton exchange membrane fuel cells (PEMFC) are expected to become a 
major source of environment-friendly energy in the near future. During the operation of a 
fuel cell, the energy producing reactions are: 
Alkaline Fuel cell: 
Anodic :   H
2
+2(OH
-
)  ------ 2H
2
O +2e
-
Cathodic : ? O
2
 + H
2
O+2e
-
   ------ 2(OH
-
) 
Polymer Electrolyte Fuel cells: 
Anodic : H
2
  ----- 2H
+
 +2e
-
Cathodic : ? O
2
 + 2H
+
 +2e
-
 ----- H
2
O 
The following efficiency diagram for a fuel cell in operation, deals with the origin of 
losses during a fuel cell reaction cycle. 
 
 
 
 
 
Fig.6. 1 Schematic diagram depicting the efficiency losses observed during the operation 
of a typical fuel cell.
1
 
From observing the typical fuel cell performance curves shown in Fig.6.1, we can 
understand that the deviation from ideal cell potential current response could be caused 
by three factors: (i) activation overpotential (lack of electrocatalysis) (ii) Ohmic losses 
and (iii) mass transport losses. Of these three factors, activation overpotential is the 
region of focus for our group and in particular, the major contributing factor is the 
oxygen reduction reaction occurring at the cathode.
2-5
 
Development of electrocatalysts for oxygen reduction reaction (ORR) has reached 
a new milestone after bimetallic effects
6-11
 of improving catalytic properties have been 
employed. These bimetallic alloys or nanoparticles have been very efficient in improving 
the activities of the individual metals and to reach the overpotentials observed in 
platinum electrodes. While the elementary steps of the ORR are not yet firmly 
established, thermodynamic analyses, such as those published recently by Bard, et al.
7, 9
 
 109
 110
and Balbuena, et al.,
12, 13
 suggest that the ORR can be viewed as consisting of an initial 
dissociative chemisorption step followed by the four-electron reduction of the oxide to 
water. 
2M + O
2
              ? 2MO            (6.1) 
2MO + 4e
?
 + 4H
+
 ? 2H
2
O + 2M   (6.2) 
Analysis of thermochemical data for these two reactions spanning a wide range of 
metals indicates that elements such as W, which form stable bonds to O, perform well for 
O
2
 bond scission and poorly for O atom reduction. Conversely, metals such as Au, which 
reduce adsorbed O atoms efficiently, are ineffective with respect to O
2
 bond scission. 
Based on these observations, a simple guideline for the development of bimetallic 
catalysts has been proposed: combine a metal that is good at O-O bond scission with a 
second metal that reduces adsorbed O atoms efficiently. A possible difficulty with this 
approach is that, in the case of metals that form stable M-O bonds, there would be a 
tendency for the O atoms generated in reaction 6.1 to remain adsorbed on the first metal, 
effectively shutting down reaction 6.2. In practice, a compromise is generally sought, as 
in the case of bimetallic catalysts such as Co-Pd, where one metal is slightly better at 
bond scission (Co) and the second is slightly better at reducing adsorbed O atoms (Pd). 
Polyoxometalates (POMs) are nanometer sized metal oxygen containing anions 
that have a wide range of catalytic applications owing to their ability to mediate 
electron,
14
 proton and oxygen atom transfer reactions. These attributes, as well as their 
ability to catalyze the reduction of hydrogen peroxide
15
,make them particularly 
interesting as oxygen reduction catalysts. Indeed, a number of research groups have 
explored the catalytic activity of POMs toward the ORR on glassy carbon electrodes as 
 111
well as on supported Pt nanoparticles
16, 17
. There are several other applications of POMs 
in fuel cells.
18-21
 , where they have been employed as electrocatalysts for both cathodes 
and anodes
18
,  and as proton conducting component in fuel cell membranes.
21, 22
 
We believe that transition metal substituted POMs adsorbed on the surface of an 
electrode that is catalytically active for the reduction of adsorbed O atoms (i.e., Au, Pd, 
Pt) should function in much the same way as a typical bimetallic ORR catalyst. In this 
contribution, we set out to design POM co-catalysts for Au, Pd and Pt electrodes using 
the thermodynamic guidelines of Bard?s model. 
In this work, we have tried to emulate the bimetallic nano-scale effects observed 
for ORR, in our polyoxometalate mediated electrocatalytic process. The scientific 
premise happens to be in the following line: A transition metal substituted 
polyoxometalate (a well characterized molecular entity), acts as an electron mediator 
between the working electrode (for e.g. Au or Pt) and the substrate to be reduced, i.e., 
dissolved oxygen. In the interface between the polyoxometalate solution and the working 
electrode, there is a localized environment, which is very similar to the bimetallic scheme 
as observed in the nanoparticles. Thus the electrocatalytic properties of the substituted 
transition metal is coupled with the electrocatalytic property of the substrate metal 
(working electrode), to accomplish the synergistic electrocatalysis observed in bimetallic 
alloys and nanoparticles. 
A systematic evaluation of the performance of different transition metal 
substituted polyoxometalates (TMSP) has been performed and an attempt has been made 
to correlate the overpotential of oxygen reduction reaction (ORR) to the thermodynamic 
free energy of metal oxygen bond formation. Beginning with the sandwich type 
 112
polyoxometalates in near neutral conditions (pH=5, owing to their stability 
considerations), we have employed the same thermodynamic considerations to further 
evaluate the performance of mono-transition metal substituted Wells-Dawson structures 
in acidic conditions (pH=1, 0.1M HClO
4
). This exercise has been performed to validate 
our claims of achieving bimetallic catalysis using polyoxometalate species in polymer 
electrolyte membrane fuel cells (PEMFC) conditions. 
 
6.2. Materials and Methods 
6.2.1. Synthesis of Wells-Dawson type polyoxometalates 
Sodium phosphotungstate hydrate, sodium tungstate and phosphoric acid were 
used as received from Sigma-Aldrich. P
2
W
18
O
62
6?
 and the corresponding ?
2
-mono 
lacunary derivative were prepared using Conant?s syntheses.
15,16
 Co-, Fe-, and Ru-
substituted Wells-Dawson compounds were prepared according to established literature 
procedures.
17-19
 PCoW
11
O
39
5? 
was synthesized from sodium phosphotungstate following 
the procedure published by Weakley, et al.
20
 Electrochemical measurements were carried 
out in a home built three-electrode glass cell with a Pt counter electrode and an 
Ag|AgCl(sat) reference electrode. The Ag|AgCl(3M NaCl) electrode was isolated from 
the working electrode using a salt bridge and was calibrated against a reversible hydrogen 
electrode (RHE); all potentials are referenced to the RHE. The electrochemical circuit 
was controlled using a BAS Epsilon electrochemical workstation. The supporting 
electrolyte was 0.10 M HClO
4
 prepared using 18.2 M? deionized water. Oxygen 
reduction measurements were carried out in saturated O
2
 solutions. 
 
 113
Equipment  
Pt electrode is 1.6mm (BAS MF 2013). Au electrode is 1.6mm (BAS MF 2014), 
Pd electrode is 2.0mm (Alfa Aesar 99.95% metals basis), GC electrode is 3.0mm (BAS 
MF 2012). 
6.3. Results and Discussion 
6.3.1 Application of bimetal substituted sandwich type structures for oxygen 
reduction in pH=5 acetate buffer 
From previous chapters, we have observed that the bimetal substituted sandwich 
type (two trivacant Keggin units connected by a central metal belt), are stable within a pH 
window of 4 to 8, hence an acetate buffer (pH=5) was chosen as the test electrolyte. 
Literature results have revealed that in the course of testing electrocatalysts for oxygen 
reduction, glassy carbon has been chosen as the working electrode of choice, since the 
base electrode does not have an inherent catalytic activity and has a large potential 
window to explore the properties of a transition metal catalyst. Oxygen reduction reaction 
at a pH 5 acetate buffer has been evaluated with the bimetallic substituted sandwich type 
polyoxometalates, with different transition metals in the central metal belt. The various 
transition metals are as follows: Ru(III), Ir, Fe, Co, Mn and Zn (Precursor). 
Fig. 6.2 reveals the oxygen reduction characteristics for the different transition metal 
substituted species (TMSP). An analysis of the figure reveals that di-iridium substituted 
sandwich compound exhibits the most positive overpotential for oxygen reduction 
reaction (ORR). The positive shift of ORR, from the potential of the bare glassy carbon 
electrode corresponds to an enhanced electrocatalytic activity due to the presence of the 
transition metal substituted POM. The general trend, in terms of ORR activity is as 
follows: Ir
2
POM > Ru
2
POM > Co
2
POM > Fe
2
POM > Zn
2
POM > Pure GC > Mn
2
POM. 
 
Fig.6. 2 Comparison of polarization curves for the oxygen reduction reaction (ORR) on 
several transition metal substituted sandwich type polyoxometalates at a glassy carbon 
electrode (3.0mM Diameter). Supporting electrolyte ? pH = 5 acetate buffer, Pt mesh as 
counter electrode, Ag/AgCl as reference electrode. Scan rate ? 10mV/s. 
 
 
 
 
 114
 
Fig.6. 3 Thermodynamic correlation between the free energy of M-O bond formation and 
the E
1/2
 for ORR at the transition metal substituted sandwich POM solutions. 
From Fig.6.3, we see that there exists a simple correlation between the position of 
the half wave potentials for oxygen reduction reaction and the enthalpy of metal oxide 
formation (eqn 6.1). The direct proportionality between the thermodynamic enthalpy of 
formation of metal oxide and the oxygen reduction wave reveals that the central metal 
atom plays an important role in this reaction. On the basis of these results, we have 
explored the opportunity of employing other transition metal substituted POMs in an 
acidic medium, which is more suited to fuel cell applications. POM structures such as 
Keggin and Wells-Dawson have been employed for our subsequent studies, because these 
compounds are more stable than sandwich type structures under the acidic conditions. 
 
 115
 
6.3.2 Application of mono-metal substituted Wells-Dawson polyoxometalates for 
oxygen reduction at acidic conditions 
 Oxygen reduction at glassy carbon electrodes were repeated with mono-
substituted Dawson type structures with different transition metal substitution. The 
choice of electrolyte was 0.1M HClO
4
 and a systematic trend seen in the sandwich type 
structures of the previous study has been observed in this study also. Further studies were 
performed on noble metal electrodes to elucidate the bimetallic effects 
 
Fig.6. 4 Comparison of polarization curves for ORR at different transition metal 
substituted Wells-Dawson POM solutions at GC (3.0mM Dia) electrode in 0.1M HClO4 
 116
solution (pH=1). Pt mesh as counter electrode and Ag/AgCl as reference electrode. 
(Reported in terms of RHE) Scanrate ? 10mV/s. 
 
s48s46s55s48s46s54s48s46s53s48s46s52s48s46s51s48s46s50s48s46s49s48s46s48
s80s111s116s101s110s116s105s97s108 s40s86 s118s115s46 s78s72s69s41
s65s117
s65s117s43s87s80s79s77
s65s117s43s70s101s80s79s77
s65s117s43s67s111s80s79s77
s65s117s43s82s117s80s79s77
s50s48s48
s49s56s48
s49s54s48
s49s52s48
s49s50s48
s49s48s48
s56s48
s54s48
s198s69s161s39
s40s109s86s41
s45s50s56s48 s45s50s52s48 s45s50s48s48 s45s49s54s48
s198s72s161
s102
s40s107s74 s47 s109s111s108s41
s87
s82s117
s67s111
s70s101
s50s181s65
 
Fig.6. 5 Voltammetric scans at an Au electrode immersed in O
2
 saturated 0.1 M HClO
4
. 
POM concentrations were [WPOM] = [FePOM] = [RuPOM] = 0.2 mM and [CoPOM] = 
0.2 ?M. Scan rate: 10 mV/sec. Inset: Plot of maximum observed ORR potential shift, 
?E?? vs. ?H?
f
 of the corresponding bulk oxide. 
 
 
 117
 118
 
Representative linear sweep voltammograms (scan rate, 10 mV/sec) showing the 
influence of a series of P
2
W
17
M
n
O
62
(12-n)?
  POMs on the electrochemical reduction of 
dioxygen at an Au electrode immersed in 0.10 M HClO
4
 are presented in Fig.6.5. Au was 
chosen as the cathode for these measurements because it is not catalytically active for O
2
 
bond scission as evidenced by an unfavorable free energy change for reaction 6.1. To 
avoid visual clutter, only the reductive sweeps are shown. The solid black curve shows 
the ORR behavior of a clean Au electrode in the absence of added POM, and is consistent 
with previous research on this system. As indicated by the dashed curves, addition of 
POM to the supporting electrolyte leads to a significant and reproducible positive shift in 
the O
2
 reduction potential (?E??). The concentration of POM added to the supporting 
electrolyte was optimized separately for each compound to give the maximum shift of the 
ORR potential in each case. ?E?? increases in the order W < Fe < Co < Ru, which 
correlates extremely well with the heats of formation (?H?
f
) of the corresponding bulk 
metal oxides (inset to Fig. 6.5).
21,22
 This behavior is in agreement with the predictions of 
the simple thermodynamic models discussed earlier, namely, that POMs with stronger M-
O bonds exhibit a weaker influence on oxygen reduction at Au surfaces (i.e., a less 
positive shift of the potential) than POMs with weaker M-O bonds. 
Next, we investigated the influence of the Wells-Dawson anions on the ORR at 
Pd, a more catalytically active electrode material. The voltammetric response of a Pd 
cathode immersed in the series of electrolytes described above is shown in Fig.6.6. 
Again, we optimized the concentration of each POM with respect to maximum ?E??. The 
behavior of the polycrystalline Pd electrode in the absence of added POM is shown as the 
 119
solid black curve for reference, and is consistent with previously published studies. When 
POM is added to the supporting electrolyte, ?E?? increases, but in a slightly different 
order (W < Fe < Ru < Co) than was seen with the Au electrode. In addition, the ORR 
potentials are observed to shift both to more positive (red curves) as well as to more 
negative values (blue curves) relative to pure Pd. The Co- and Ru-substituted Wells-
Dawson anions improve the performance of the Pd cathode, while the Fe-substituted 
POM as well as the un-substituted Wells-Dawson anion (WPOM) shift the ORR potential 
to significantly more negative potentials (i.e., lead to an increase in the ORR 
overpotential). In the case of Pt cathodes, addition of W-, Fe- and Ru-containing POMs 
to the supporting electrolyte resulted in substantial negative potential shifts at all 
concentrations investigated (data not shown). Negative shifts of the O
2
 reduction 
potential can be rationalized on the basis of site blocking effects that influence the 
kinetics of the ORR on Pd and Pt electrodes (e.g., by inhibiting the formation of the metal 
oxide). Similar observations have been reported recently in the case of phosphotungstate 
and phosphomolybdate anions adsorbed on Pt nanoparticles.
13
  
If the negative potential shifts observed for Pd and Pt electrodes are indeed due to 
surface effects such as active site blocking, changing the concentration of the POM in the 
supporting electrolyte is expected to influence the magnitude of the shift by altering the 
adsorption dynamics/equilibrium. Concentration dependent measurements using the Co-
substituted Wells-Dawson POM with a Pt cathode strongly support this hypothesis, 
Fig.6.7. This data set shows the effect of [CoPOM] on the ORR response of a Pt 
electrode immersed in 0.1 M HClO
4
. The solid black curve shows the behavior of a 
naked Pt electrode in the absence of added POM. When the concentration of Co-POM is 
 120
increased from about 2 ?M to 0.2 mM, the ORR curves shift to more negative values, as 
illustrated by the dashed blue traces. For POM concentrations below about 2 ?M, the 
trend reverses, and ?E?? increases slightly as the POM concentration increases, as 
illustrated by the red curves. The maximum ?E?? occurs when [CoPOM] = 0.2 ?M. 
Under these experimental conditions, the ORR potential is shifted 21 mV positive of that 
observed at a naked Pt electrode. A plot of the voltage shift as a function of concentration 
for the complete data set is shown in the inset to Fig.6.7.  
Finally, in an effort to minimize active site blocking effects, we decided to 
investigate the behavior of Keggin ions as co-catalysts, since we have found them to be 
less strongly adsorbing on metal surfaces compared to the Wells-Dawson series. 
Representative voltammetric traces in the ORR potential region are shown in Fig.6.8 for 
a Pt electrode immersed in a series of POM containing electrolytes. Traces corresponding 
to unsubstituted PW
12
O
40
3? 
are shown in blue, while data corresponding to the Co-
substituted Keggin (PCoW
11
O
39
5?
) are shown in red. The presence of PW
12
O
40
3?
 leads to 
a negative shift of the ORR potential for all concentrations studied. On the other hand, 
addition of PCoW
11
O
39
5?
 to the background electrolyte leads to substantial positive shifts 
in the potential. The fact that the optimized ?E?? value observed for PCoW
11
O
39
5?
 (20 
?M) is significantly larger (+54 mV) than what was observed in the case of the analogous 
Wells-Dawson ion is consistent with our hypothesis that site blocking effects are less 
important for the more weakly adsorbed Keggin ions. 
 
Fig.6. 6 The voltammetric response of a Pd electrode immersed in O
2
 saturated 0.1 M 
HClO
4
. POM concentrations were [WPOM] = [FePOM] = [RuPOM] = 0.2 mM and 
[CoPOM] = 0.2 ?M. Scan rate: 10 mV/sec.  
 
 
 
 121
 
Fig.6. 7 The voltammetric response of a Pt electrode in O
2
 saturated 0.1 M HClO
4
 as a 
function of [CoPOM]. Scan rate: 10 mV/sec.  Inset: plot of ?E?? versus [CoPOM]. 
 
 
 122
 
Fig.6. 8 The voltammetric response of a Pt electrode in O
2
 saturated 0.1 M HClO
4
 
containing the noted concentrations of PW
12
O
40
3? 
and PCoW
11
O
39
5?
 Scan rate: 10 
mV/sec. Inset: plot of ?E?? versus [CoPOM]. 
In conclusion, transition metal substituted Wells-Dawson POMs adsorbed on the 
surface of Au electrodes lead to positive shifts of the ORR potential by a mechanism 
analogous to that which has been proposed for bimetallic ORR catalysts. The magnitude 
of the shift can be quite large (ca. 200 mV), is transition metal dependent, and can be 
explained using simple thermodynamic concepts. In the case of more active cathode 
materials such as Pd and Pt, our results indicate that surface effects become increasingly 
important: there is an optimum concentration of POM for which catalytic activity and 
site-blocking effects are in balance. In the case of Pd cathodes a maximum ?E?? of nearly 
 123
 124
+100 mV was observed. The best overall cathode performance was achieved using 
PCoW
11
O
39
5?
 in conjunction with a Pt electrode (+54 mV positive shift compared to bare 
Pt), and this result is comparable to the performance of bimetallic catalysts.
3
 Finally, the 
approach to enhancing the performance of oxygen reduction cathodes that we have 
described is simple to implement and is compatible with existing fuel cell cathode 
materials. 
References 
1. Appleby, A. J.; Foulkes, F. R., Fuel Cell Handbook. Van Nostrand Reinhold: NY, 
   1989. 
2. Jitianu, M.; Kleisinger, R.; Lopez, M.; Goia, D. V.,Journal of New Materials for  
   Electrochemical Systems 2007, 10, (2), 67-74. 
3. Mustain, W. E.; Kepler, K.; Prakash, J.,Electrochimica Acta 2007, 52, (5), 2102-2108. 
4. Travitsky, N.; Ripenbein, T.; Golodnitsky, D.; Rosenberg, Y.; Burshtein, L.; Peled, 
    E.,Journal of Power Sources 2006, 161, (2), 782-789. 
5. Markovic, N. M.; Ross, P. N.,Surface Science Reports 2002, 45, (4-6), 121-229. 
6. Fernandez, J. L.; White, J. M.; Sun, Y. M.; Tang, W. J.; Henkelman, G.; Bard, A. 
    J.,Langmuir 2006, 22, (25), 10426-10431. 
7. Walsh, D. A.; Fernandez, J. L.; Bard, A. J.,Journal of the Electrochemical Society  
   2006, 153, (6), E99-E103. 
8. Fernandez, J. L.; Raghuveer, V.; Manthiram, A.; Bard, A. J.,Journal of the American  
   Chemical Society 2005, 127, (38), 13100-13101. 
9. Fernandez, J. L.; Walsh, D. A.; Bard, A. J.,Journal of the American Chemical Society  
   2005, 127, (1), 357-365. 
 125
10. Fernandez, J. L.; Bard, A. J.,Analytical Chemistry 2003, 75, (13), 2967-2974. 
11. Liu, B.; Bard, A. J.,Journal of Physical Chemistry B 2002, 106, (49), 12801-12806. 
12. Wang, Y. X.; Balbuena, P. B.,Journal of Physical Chemistry B 2005, 109, (40),  
      18902-18906. 
13. Balbuena, P. B.; Altomare, D.; Vadlamani, N.; Bingi, S.; Agapito, L. A.; Seminario, 
     J. M.,Journal of Physical Chemistry A 2004, 108, (30), 6378-6384. 
14. Keita, B.; Mbomekalle, I. M.; Lu, Y. W.; Nadjo, L.; Berthet, P.; Anderson, T. M.;  
      Hill, C. L.,European Journal of Inorganic Chemistry 2004, (17), 3462-3475. 
15. Toth, J. E.; Melton, J. D.; Cabelli, D.; Bielski, B. H. J.; Anson, F. C.,Inorganic 
      Chemistry 1990, 29, (10), 1952-1957. 
16. Chojak, M.; Kolary-Zurowska, A.; Wlodarczyk, R.; Miecznikowski, K.; Karnicka, 
       K.; Palys, B.; Marassi, R.; Kulesza, P. J.,Electrochimica Acta 2007, 52, (18), 5574- 
       5581. 
17. Wlodarczyk, R.; Kolary-Zurowska, A.; Marassi, R.; Chojak, M.; Kulesza, P. 
      J.,Electrochimica Acta 2007, 52, (12), 3958-3964. 
18. Limoges, B. R.; Stanis, R. J.; Turner, J. A.; Herring, A. M.,Electrochimica Acta 2005, 
     50, (5), 1169-1179. 
19. Geletii, Y. V.; Gueletii, A.; Weinstock, I. A.,Journal of Molecular Catalysis a- 
     Chemical 2007, 262, (1-2), 59-66. 
20.Justi, E. W. W., E. A., Kalte Verbrennung-fuel Cells. Franz Steiner Verlag,  
    Wiesbaden, : 1962; p 248. 
21. Kuo, M. C.; Stanis, R. J.; Ferrell, J. R.; Turner, J. A.; Herring, A. M.,Electrochimica 
     Acta 2007, 52, (5), 2051-2061. 
 126
22. Jennifer, L. M.; Sweikart, M. A.; Horan, J. L.; Turner, J. A.; Herring, A. M.,Journal 
     of Power Sources 2007, 172, 83-88. 
 
 
 127
CHAPTER 7 
ANTIBACTERIAL APPLICATIONS OF SANDWICH TYPE 
POLYOXOMETALATES 
7.1. Introduction 
In the course of history, different kinds of epidemics have wreaked havoc on 
human lives and disease causing microorganisms have been the center of all these 
turmoil. In recent times, the development of potent antibiotics have been responsible for 
keeping these organisms at bay, but nature has been silently at work in evolving counter 
measures. Antibiotics resistant bacterial strains have been discovered in a number of 
infections and research has been targeted towards development of more effective and 
non-conventional antibacterial drugs. In this regard, polyoxometalates have been targeted 
as antibacterial agents against methicillin resistant Staphylococcus aureus
1
, Helicobacter 
pylori
2
 and several other bacteria. Yamase et al
3
 have shown that ?-lactam resistant 
bacterial strains can be treated with different polyoxometalate species along with 
traditional antibiotics. The existence of synergistic effects between traditional antibiotics 
and polyoxometalates has been established by this group.  
In this work, we propose to use our expertise in electrochemical characterization 
of polyoxometalates to correlate the redox properties of these unique molecules to their 
observed antibacterial activity. Two distinct types of polyoxometalates, a Keggin 
structure (PVW
11
O
40
) and a sandwich type  [Na
12
{V
2
Zn
2
(ZnW
9
O
34
)
2
}], have been chosen 
 128
as therapeutic agents to be tested on two bacterial species Listeria monocytogenes and 
Salmonella typhimurium. These species were chosen because of high level of 
contamination associated with poultry products.  
7.2. Antibacterial Properties of Polyoxometalates  
The biological application of POMs were initially demonstrated in France
3
, when 
certain POM species [SiW
12
O
40
4-
, Na(SbW
7
O
24
)
3
(Sb
3
O
7
)
2
18- 
and (AsW
9
O
33
)
4
(WO
2
)
4
28-
] 
were tested for their activity against non-retro RNA and DNA viruses, together with the 
study of inhibition of Friend leukemia virus using SiW
12
O
40
4- 
and other POMs. According 
to Hill, the antiviral and anti-tumoral activities of POMs have been extensively studied in 
the pharmaceutical industry, while the third area of antibacterial application is slowly 
gaining importance. The antibacterial properties of polyoxometalates were 
serendipitously discovered in the laboratory of Tajima et al
4
 in 1993. Initially the species 
responsible for enhancing the efficiency of ?-lactum antibiotics on methicillin-resistant 
Staphylococcus aureus (MRSA) was thought to be a ?Factor T?, which was subsequently 
found to be the lacunary Keggin species, PW
11
O
39
 
7-
. These Keggin species are known to 
increase the potency of ?-lactum antibiotics on resistant bacterial strains by mainly 
suppressing penicillinase production and reducing the formation of penicillin-binding 
protein-2?. It was also shown to cause a metachromatic reaction and to prolong the blood 
coagulation time, thus providing some evidence of heparin-like polyanion behavior. The 
same study proposed that since these compounds become inefficient in the presence of 
polycationic species, an electrostatic charge interaction plays a major role in the 
antibacterial enhancement process. Based on these observations, several research groups 
have started evaluations focusing on the redox-active nature of the polyoxometalates to 
 129
be part of the driving force in drug-like activities. There are studies showing that these 
polyoxometalates can non-specifically inhibit several anion-sensitive enzymes ? most 
nucleotide interacting enzymes, but not serum enzymes
4
.   
Yamase et al 
1, 3-6
 have studied the anti-bacterial activity of antibiotics such as 
oxacillin or penicillin (?-lactam compounds) in synergy with a variety of 
polyoxometalates from Keggin, lacunary Keggin, Wells-Dawson, to double Keggin 
(Sandwich compound)
6
. They have shown that both Gram-positive & Gram-negative 
bacteria are affected by these compounds. Specifically, they have addressed the 
enhancement effect of POMs on methicillin resistant Staphylococcus aureus (MRSA) and 
vancomycin resistant Staphylococcus aureus (VRSA), which are Gram-positive bacteria.  
?-lactum resistance in MRSA has been associated with the production of an additional 
peptidoglycan-synthetic enzyme called the penicillin-binding protein 2? (as mentioned 
above). This protein is responsible for resistance against ?-lactam antibiotics, by forming 
a protective peptidoglycan layer on the cell wall. Systematic studies using gel-
electrophoresis have revealed that polyoxotungstates suppressed the formation of this 
protein which is essential for cell wall generation during the MRSA growth process. The 
polyoxotungstates are also shown to reduce the production of pencillinase or ?-lactamase, 
the enzyme produced in the membrane which hydrolyzes ?-lactam antibiotics and makes 
them ineffective. Further studies by the same group have revealed the localization of 
polytungstate (P
2
W
18
O
62
6-
) species at the cell wall of VRSA. (Using TEM images and 
energy dispersive X-ray analysis) 
A particularly interesting phenomenon of biological reduction of polytungstates 
(P
2
W
18
O
62
6- 
, E
1/2 
= +0.058V & SiMo
12
O
40
4-
, E
1/2
 = +0.237V) has been observed in 
 130
MRSA and VRSA cell cultures and this has been claimed as evidence for penetration of 
POMs through the cell wall to reach the cytoplasmic membrane. Structural integrity of 
the polyoxotungstates has been shown to be important for enhancement and hence 
species stable in near neutral physiological conditions are ideal drug candidates.  
Other than enhancement of antibiotic activity, there are several instances where 
the POM species have shown anti-bacterial activity on their own accord. Highly 
negatively charged polytungstates such as Keggin (SiVW
11
O
40
5-
) and other sandwich-
type species [(AsW
9
O
33
)
4
(WO
2
)
4
28-
] have been employed against Helicobacter pylori
7
 
(Gram-negative bacteria) 
 The redox property of medicinal drugs has been a field of exploration for a long 
time now. Polyoxometalates with sandwich type structures have good stability in neutral 
conditions, which is close to the physiological condition and hence these were chosen for 
our studies in antimicrobial drug development. 
7.3. Toxicity 
Transition metal based complexes have been employed as therapeutic agents for a 
long time. For any compound to be successfully tested for clinical trials, toxicity studies 
play a major role in determining relevance and application. In polyoxometalates, there is 
evidence for polyoxotungstates to bind with proteins, alkaloids etc and alter their 
functions and properties. Despite being purely inorganic in nature, tungsten compounds 
in polyoxometalates are known to have unique biological effects. One study in particular 
has addressed the effect of lacunary Keggin compounds, phosphotungstate (PW
11
O
39
7-
)  
and silicotungstate (SiW
11
O
39
8-
) on stress promoters in Escherichia coli
5
. It has been 
shown that the phosphotungstate produces an osmotic signal at lower concentrations 
 131
without increasing ionic strength by inducing the stress promoter gene, osmY (which is 
sensitive to osmotic signals). Silicotungstate was shown to induce gene clpB, which is an 
analog to human heat shock protein.  
In terms of practical applications in medicine, Tajima et al
8
 have studied the 
impact of direct administration of polyoxometalate solutions to animals. Lacunary 
Silicotungstate (SiW
11
O
39
8-
) species were injected into the veins of animals and severe 
cases of pulmonary embolism have been observed, and this has been attributed to platelet 
aggregation caused by these POMs. More studies are needed to understand the toxicity 
effects of POMs. In the case of anti-tumoral activity, these POMs are known to 
selectively accumulate and attack the diseased cells with very minimal effect on healthy 
animal cells. 
7.4. Materials and Methods  
7.4.1. Bacteria 
Staphylococcus aureus (ATCC SA), Salmonella typhimurium (ATCC 13311 ST) and 
Listeria monocytogenes (ATCC 19115 LM) were obtained from American Type Culture 
Collection (ATCC, Manassas, VA, USA). These strains were cultured in the respective 
selective agar culture medium at 35?C for 24 hrs. These agar plate cultures were stored in 
a refrigerator at 0?C and were employed as stock cultures for all the experiments.  
7.4.2. Polyoxometalate and Reagents 
Zn
2
POM precursor was prepared as described in previous chapter. Vanadium substituted 
sandwich type POM was prepared by adding vanadyl sulfate solution to Zn
2
POM 
precursor solution in the ratio of 3:1. On continuous stirring at a temperature of 90?C, a 
greenish brown solution was obtained, which on cooling produced a precipitate of the 
 132
same color. On further recrystallization with water and in the presence of a concentrated 
solution of potassium nitrate, needle like crystals were obtained. Vanadium substituted 
Keggin structure (PVW
11
O
40
5-
) was also prepared according to literature methods
9
, and 
was employed as the control species to compare the behavior of sandwich POMs. The 
bacteriostatic properties of the selected POMs were characterized by monitoring the cell 
growth in pH7 phosphate buffer media, before employing the generic growth media. All 
electrochemical experiments were performed in the same pH =7 buffer solution. 
7.4.3. Antimicrobial Assay - Qualitative Method: 
Spotting or Zone of Inhibition 
An initial concentration (10
6
 to 10
9
 CFU/ml) of bacterial cells was spread on 
selective culture plate followed by addition of 10?L of polyoxometalate on the surface. 
After incubating for 24 hrs at respective temperatures (LM at 30?C, ST and SA at 37?C) 
the plates were analyzed for inhibition zones due to POMs bacteriostatic activity. A 
similar inhibition zone study was carried out using standard microdiscs saturated with 
POMs. Paper micro discs (Fisher) were saturated with a given concentration of 
autoclaved polyoxometalate solution and added on top of an agar plate, which were 
previously plated with bacterial strains. As previously, the plates were incubated at 
required temperature for about 24 hours and the zone of inhibition was analyzed for each 
POM species. 
7.4.4. Antimicrobial Assay - Quantitative Method: 
Time Killing Assay 
The bacteriostatic effects of POMs were evaluated by mixing a known 
concentration of POM to starting bacterial culture.  At different time intervals, a 100 ?l of 
 133
solution was withdrawn and plated on an agar plate and incubated at respective 
temperature for 24 hrs. Control experiments were performed from bacterial solution 
containing no POMs. Finally, number of colonies formed on the plate was counted to 
quantify the bacteriostatic effect of POMs. 
Viability of bacterial cells has been determined by preparing a 10-fold serial 
dilution of bacterial species in phosphate buffered saline solution and then about 100 ?l 
aliquot was inoculated onto agar plates. Initially a known concentration of bacterial cells 
placed in PBS saline and then a known concentration of POM solution (drug) was added. 
The idea is to identify the nature of bacteriostatic activity and hence the broth is 
immediately diluted in a successive manner to about 10 orders of magnitude from the 
initial concentration and each diluted concentration is saved in an individual test-tube. 
The diluted broth is then plated onto agar culture medium and subjected to incubation for 
24-48 hours, to count the colony forming units, as a function of dilution. The diluted 
broth solutions are themselves placed in an incubator and 100 ?l aliquots are removed as 
a function of time (every 2hrs) and then plated onto agar culture medium. This method of 
collecting samples as a function of time and dilution will provide the time killing assay or 
bacteriostatic curve for each population. The absence or the reduction in number of 
colonies in drug treated medium implies inhibition of bacterial growth. 
7.4.5. Characterization of Redox Activity of Inhibitive Polyoxometalates 
Cyclic voltammetry of L-cysteine oxidation was performed in the presence of the 
different polyoxometalates to provide a simple method to establish oxidation as a route to 
bacterial inhibition. All experiments were performed with glassy carbon as working 
electrode, Pt mesh as counter electrode and Ag/AgCl as reference electrode. The 
electrolyte solution was a phosphate buffer solution (pH=7) and it was chosen to be 
consistent with the growth medium used in bacterial cultures. 
 
 
7.5. Results and Discussion 
7.5.1. Electrochemical Characterization 
 
Fig.7. 1  Cyclic voltammetry comparison of L-cysteine oxidation at V keggin, Zn
2
POM 
and V
2
POM 
 
The redox capabilities of the three compounds were tested using L-cysteine as the 
organic substrate. Sweeping the potential from 0 V to the positive direction reveals some 
interesting behavior in the different POM solutions. A concentration of 0.2 mM was 
chosen for all three model compounds, V Keggin (monovanadium), Zn
2
POM precursor 
and V
2
POM sandwich. L-cysteine oxidation occurs at very high oxidation potential in the 
glassy carbon electrode employed in the absence of electrocatalyst. In the presence of 
Zn
2
POM, the oxidation peak begins at about 1.05 V. In the presence of V Keggin 
 134
 135
structure, the origin of the oxidation peak occurs at 0.7V, which could correspond to 
some electrocatalysis by the vanadium metal substitution. In the presence of V
2
POM, the 
cysteine oxidation begins more closer to zero, at 0.45 V. An analysis of the oxidation 
peaks establishes that the V
2
POM sandwich exhibits better electrocatalytic property for 
cysteine oxidation. This electro-organic oxidation can be regarded as a simplistic 
indication of the potency of the polyoxometalate species with respect to the redox 
activity. 
7.5.2. Qualitative Method 
Staphylococcus aureus 
Zone of inhibition using paper disc impregnated with POMs was chosen as an 
initial screening method to establish the bacteriostatic activities of synthesized POMs. 
Initially, vanadium substituted Keggin (PVW
11
O
39
5-
) species was tested with 
Staphylococcus aureus to understand the bacteriostatic activity and compared with 
reported literature results. Under the given laboratory conditions we were able to repeat 
the results obtained by Yamase
3
 for V Keggin structure as shown in Fig.7.2. This 
experiment proved the validity of our paper microdisc method in achieving the qualitative 
analysis for the screening of polyoxometalates. Fig.7.2. also reveals the zone of inhibition 
observed in Staphylococcus aureus loaded agar plate, along with three different control 
samples. Citrate stabilized silver (Ag) nanoparticles were one of the control samples, 
since Ag has been shown to be bactericidal
10-12
 in the nanoparticle regime. The V Keggin 
saturated microdisc exhibited the largest zone of inhibition compared to V sandwich and 
Ag nanoparticles. After carefully analyzing literatures
13, 14
, a concentration of 0.2 mM 
was selected to test for bacteriostatic activities. 
 
Fig.7.2 Microdisc method of qualitative analysis of bacteriostatic activity for 
Staphylococcus aureus strain in the presence of silver nanoparticles, V Keggin and V 
sandwich polyoxometalate. 
The above testing was repeated with bacterial stains of interest, Salmonella 
typhimurium and Listeria monocytogenes using the paper disc method. In order to 
establish a minimum inhibitory concentration (MIC), the bacteriostatic activity of 
precursor compound Zn
2
POM were tested under different concentrations with 
Staphylococcus aureus and the results were shown in Fig.7.3. 
 
 
 
 
 136
 
Fig.7. 3 Bacteriostatic activity of the precursor compound Zn
2
POM, in an agar medium 
containing a 10
5
 to 10
7
 CFU/ml of Staphylococcus aureus (S.A) 
The reasons for choosing the precursor Zn
2
POM were two-fold, one to achieve a 
comparable baseline of bacteriostatic activity for the vanadium substituted sandwich and 
the other to test the effect of zinc in bacterial systems, as the zinc based compounds are 
reported to be benign at low concentrations under physiological conditions. The results 
from Fig.7.3 reveal that the minimum inhibitory concentration for Zn
2
POM, is very high, 
in the range of 2mM to attain a comparable inhibition zone to vanadium Keggin structure 
in the previous studies shown in Fig.7.2. 
Salmonella typhimurium 
The Salmonella species have been contributing to increased amounts of morbidity 
and mortality throughout the world. Further the antibiotic resistant strains have started 
thriving in the developing countries.  
 
 137
0.1 mM Zn
2
POM0.5 mM Zn
2
POM
2 mM Zn
2
POM
1 mM Zn
2
POM
 
Fig.7. 4 Microdisc method for the study of bacteriostatic effect of Zn
2
POM on 
Salmonella typhimurium (S.T) 
The effect of Zn
2
POM saturated paper discs at different concentrations were 
studied as shown in Fig.7.4. Even at a very high concentration of 2 mM, a very small 
zone of inhibition is observed. The above figure reveals that Zn
2
POM cannot be 
employed as a bactericidal agent in the case of S.T. This finding further reiterates the 
difference in the mechanism of bacterial cell growth between S.T and S.A.  
S.T exhibited a typical growth pattern in the normal control solution as shown in 
Fig.7.5. In the presence of V sandwich, there was a significant zone of inhibition, when 
compared to the precursor molecule Zn
2
POM. V Keggin on the other hand, did not 
exhibit very good inhibition, this could be attributed to the different nature of Salmonella 
when compared to Staphylococcus species and the presence of two vanadium atoms 
could be considered as detrimental to the former species. Based on this qualitative 
 138
microdisc method, the vanadium di-substituted sandwich compound can be chosen as a 
suitable candidate to carry out the other quantitative experiments. 
 
Fig.7. 5 Microdisc method to study the bacteriostatic effect of POMs on 10
5
 to 10
7
 
CFU/ml Salmonella typhimurium 
Listeria monocytogenes 
Listeria monocytogenes is a Gram-positive bacterium, is a significant pathogen 
contaminating the poultry products. It is one of the primary candidates in causing severe 
meningitis in infants and has been known to inhabit the human gastrointestinal tracts. 
Microdisc method studies employing the different concentration of Zn
2
POM against this 
pathogen (Fig.7.6) reveals that there is a significant zone of inhibition observed at both 
1mM and 2mM concentration levels. This preliminary response to Zn2POM could 
contribute towards more refinement of concentration dependence in the Listeria 
monocytogenes species, in the future. 
 139
 
Fig.7. 6 Microdisc method elucidating the bacteriostatic property of Zn
2
POM under 
different concentrations 
Finally, the susceptibility of Listeria to different POMs were performed and 
shown in fig.7.7. Similar to Salmonella, only V Keggin and V sandwich compounds 
exhibits bacteriostatic activity while little to no activity was observed with Zn
2
POM. 
 140
 
Fig.7. 7 Microdisc method to evaluate the bacteriostatic activity of Vanadium POMs and 
precursor in Listeria monocytogenes agar culture plate 
7.5.3. Quantitative method ?Time killing assay  
The time killing assay in the presence of 0.2mM of polyoxometalate species has 
been used as the quantitative method of determining the efficacy of bacteriostatic 
activity. Typically, if an agent exhibits three-orders of log reduction in bacterial 
population then it is considered as bactericidal in comparison to being bacteriostatic 
where it would limit the bacterial growth. 
Fig.7.8 and 7.9 outlines the susceptibility of Listeria monocytogenes and 
Salmonella typhimurium growth in the presence of the same concentration of V Keggin 
and V sandwich, respectively. Similar to zone-inhibition studies, V Keggin shows the 
bacteriostatic effect by preventing the growth of both bacteria even after 24 hrs. 
However, marked differences were observed between V Keggin and V sandwich as the 
latter compound reduces bacterial concentration by nearly 3 orders of magnitude, 
demonstrating bactericidal activity. As expected, time-killing assay unambiguously 
 141
distinguished bacteriostatic compounds from bactericidal ones which were not prominent 
in zone-inhibition studies.  
5
6
7
8
9
0 1020304050
Time in hrs
l
og numbe
r
 of c
e
l
l
s
V Keggin
LM Control
V Sandwich
 
Fig.7.8. Time killing assay analysis of Listeria monocytogenes as a function of exposure 
to V Keggin and V sandwich POMs. 
 
3
4
5
6
7
8
9
0 1020304050
Time in hrs
l
o
g numbe
r
 of
 c
e
l
l
s
V Keggin
ST control
V Sandwich
 
Fig.7.9.Time killing assay analysis of Salmonella typhimurium as a function of exposure 
to V Keggin and V sandwich POMs. 
 142
 143
Reasons for Antimicrobial Activity of Polyoxometalates 
In this exercise to establish the concept of antibacterial application of sandwich 
type polyoxometalate, a preliminary effort has been made to provide qualitative 
information regarding the feasibility of these POMs. From previous chapters, we have 
seen that the sandwich type polyoxometalates are very good electron transfer mediators 
and more particularly with respect to oxygen. When a redox active agent such as a POM 
is chosen as a drug candidate, there are several possibilities for attacking the cellular 
redox environment of a living cell
15
. These redox active sandwich type POMs could act 
as efficient agents to create reactive oxygen or nitrogen species (ROS, RNS). In addition, 
these POMs are known to accumulate in cell walls and lead to oxidative stress in the 
bacterial cell
1, 3, 13
 by producing reactive oxygen species such as superoxides, whose 
direct products such as H
2
O
2
, peroxynitrites and OH? which can disrupt the metabolic 
pathways by interfering with the intermediates. The biological reduction of 
polyoxometalates has been observed in bacterial cell cultures and this can be an evidence 
for the electron transfer mediation of POM in the biochemical system. Despite the fact 
that detailed study on the mode of action of sandwich type POMs on ST and LM have not 
been performed, the previous studies have revealed that redox activity could contribute to 
creating oxidative stress in bacterial cells and could cause cell apoptosis or cell death
2, 6, 7
. 
This qualitative investigation of antibacterial activity of sandwich type polyoxometalates 
can be employed as an effective launching pad for more elaborate mechanistic studies in 
the future. 
 
 
 144
References: 
1. Yamase, T.; Fukuda, N.; Tajima, Y.,Biological & Pharmaceutical Bulletin 1996, 19,  
    (3), 459-465. 
2. Miyao, I.; Keiko, S.; Sae, M.; Nobuhiro, M.; Mayumi, O.; Toshihiro, Y.,Journal of  
    inorganic biochemistry 2005, 99, (5), 1023-31 
3. Yamase, T.,Journal of Materials Chemistry 2005, 15, (45), 4773-4782.  
4. Y.Tajima; Z.Nagasawa; J.Tadano,Microbiol. Immunol 1993, 37, 695. 
5. Y.Tajima,Journal of inorganic biochemistry 2003, 94, 155. 
6. T.Yamase; N.Fukudo; Y.Tajima,Biological & Pharmaceutical Bulletin 1996, 19, 459. 
7. S.Ikeda; S.Nishiya; A.Yamamoto; T.Yamase; C.Nishimura; Clercq, E. D.,J. Med. Virol 
   1993, 41, 191. 
8. Tajima, Y.,Biological & Pharmaceutical Bulletin 2001, 24, (9), 1079. 
9. Keita, B.; Mbomekalle, I. M.; de Oliveira, P.; Ranjbari, A.; Justum, Y.; Nadjo, L.; 
   Pompon, D.,Journal of Cluster Science 2006, 17, (2), 221-233. 
10. Pohle, D.; Damm, C.; Neuhof, J.; Rosch, A.; Munstedt, H.,Polymers & Polymer 
     Composites 2007, 15, (5), 357-363. 
11. Shahverdi, A. R.; Fakhimi, A.; Shahverdi, H. R.; Minaian, S.,Nanomedicine- 
     Nanotechnology Biology and Medicine 2007, 3, (2), 168-171. 
12. Lee, H. Y.; Park, H. K.; Lee, Y. M.; Kim, K.; Park, S. B.,Chemical Communications  
    2007, (28), 2959-2961. 
13. Inoue, M.; Suzuki, T.; Fujita, Y.; Oda, M.; Matsumoto, N.; Yamase, T.,Journal of  
    Inorganic Biochemistry 2006, 100, (7), 1225-1233. 
14. Kong, Y. M.; Peng, J.; Xue, B.; Pan, L. N.; Xin, Z. F.; Li, L.; Lu, J.,Chemical Journal 
 145
      of Chinese Universities-Chinese 2006, 27, (5), 801-804. 
15. Hillard, E. A.; Abreu, F. C. d.; Ferreira, D. C. M.; Jaouen, G.; Goulart, M. O. F.;  
     Amatore, C.,Chemical Communications 2008, (23), 2612-2628. 
 
 
 146
CHAPTER 8 
CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK 
The primary objective of this research work was to develop new electrocatalysts 
using the versatile polyoxometalate structures. We have been successful in synthesis and 
characterization of transition metal substituted POMs for different applications such as 
oxygen generation, oxygen reduction and electro-organic oxidation reactions.  
In terms of water oxidation, a novel di-ruthenium substituted sandwich type 
polyoxometalate has been demonstrated to generate oxygen at potentials very close to 
thermodynamic limits. This is one of the first examples of inorganic ruthenium 
complexes to provide opportunities for more efficient catalytic systems than existing 
systems. During the course of electrocatalytic studies, an adsorption phenomenon has 
been observed for the sandwich type POMs, onto gold and glassy carbon electrodes. This 
absorption under the influence of electrochemical potential cycling has been studied 
using electrochemical surface plasmon resonance. As a result of this study, an elegant 
method to complement the mass change studies using electrochemical quartz crystal 
microbalance studies has been proved for POM adsorption. The electrochemical potential 
stability window for the case of Ru
2
POM and Zn
2
POM has been obtained using this 
study. It has also been shown that sandwich type POM undergoes a higher rate of 
adsorption than Keggin species under the same conditions of concentration and potential 
window. Future studies are needed to understand the mechanism of oxygen generation 
 147
from these diruthenium POM structures and to elucidate the catalytically active species in 
these reactions. This can be accomplished by the employment of in-situ Raman during 
electrochemical studies of the interface. These studies could also reveal the nature of the 
adsorbed species on the electrode surface during electrochemical cycling. 
Novel di-iridium substituted sandwich type POMs have been synthesized and 
characterized for the first time. These structures have been employed for both 
electrocatalytic oxidation and reduction reactions. The potential for employment of these 
structures as electrocatalysts for regenerative fuel cells and as sensing electrodes for 
biological species such as cysteine and ascorbic acid can be explored in the future. 
In the direction of oxygen reduction, a simple thermodynamic model proposed for 
bimetallic electrocatalysis has been shown to exist for polyoxometalate co-catalysts in the 
presence of noble metal electrodes. A significant improvement was achieved for oxygen 
reduction potential at palladium electrodes (+100 mV) and platinum electrodes (+54 mV) 
and these values correspond to the values of the most efficient bimetallic electrocatalysts 
existing today. Future studies to elucidate the kinetics of electron transfer have been 
recommended to be performed using rotating disc electrode studies. Further studies 
should also focus on the immobilization of these homogeneous electrocatalysts onto 
electrode surfaces to achieve transition onto real-world applications. 
Finally, the sandwich type POMs have been demonstrated to exhibit unique anti-
bacterial applications on Salmonella typhimurium and Listeria monocytogenes. These 
preliminary studies could be extended onto other bacterial species and the nature of the 
bactericidal action also needs more exploration.